Depends. I like both games honestly, some might not.
Fallout 3 has a reputation of being the same as recent (at the time) release, TES4: Oblivion, both games being developed under Bethesda Studios, with a change of scenery and guns added. But it really conveys wonderfully the feel of a destroyed wasteland of a world, a city that meant so much in the entire world being demolished to little more than rubble and nuclear waste (In this case of course, Washington DC and the surrounding couple square miles, I think 10?). It's an honest replication of the feeling of the game's story world, though it's worth keeping in mind this is a place that was hit far harder than California, where all the other titles thus far have taken place. Unexploded nukes still rest in the landscape, slavery and mutant attacks are natural and common, while the remaining standing armed forces vie for any sort of control possible in a world that has little more than free-standing settlements. And as a site of history, you'll find many interesting places in the world from old-time guns to a historical museum of flight. The game however is easier than Fallout 1,2, or New Vegas, and part of that is the change into full 3D instead of Isometric Perspective, and with that came free-aiming instead of having to use the VATS targeting system. A person who's put even a few points into any sort of gun can find themselves an expert shot with a little practice, where I myself could manage to take out most human characters at RIFLE distance using the classic shotgun (I'm not sure shotguns ever used bullet wheels, but it's an interesting touch), and using the basic Hunting Rifle was enough to kill Super Mutants at a couple shots without being hit. The sneak-attack bonuses make this even more incredible, and when you combine Headshot, Sneak-Attack, and with that much damage, Cripple Damage Stagger, it becomes almost methodical to beat most enemies out in the open. In short distance, I boosted my sneak skill, stayed in the shadows, and could take out any and all mutants in about two shots to the head with VATS, since it is a bit hard to fire at a surprised target that close. The only people in game I had trouble with were actually the robots, who had little targets, rockets on regular occasion, and were difficult to cripple in any one area (Targeting, Inhibition Chip, Weapon, Mobility), but I had enough grenades and carried so many mines that I could get around with weakening them and finishing them off with a rail-gun (Has to be made). Finally, even though it is a wasteland, a character with time and effort can find themselves able to buy the moon if they wanted to. It's not a hard game so long as you put decent focus into any one type of gun, my favorite being rifles (Small Arms), and so much money can collected that you can repair armor and gun infinite without worry, even if you only gather money by killing the various mutants and collecting their hunting rifles, assault rifles and shotguns to sell, ignoring powerful money making missions like book collecting for the Brotherhood of Steel or ravaging buildings for pre-war money, cigarettes, and selling all the bullets and ammo you don't plan on using since again, with enough practice and experience (And no ammo weight) you only need 2 guns, in my case a hunting rifle (Or Lincoln Rifle, or Railgun, depending on ammo) and a shotgun, with ion weapons for back-up.
Fallout New Vegas, developed by Obsidian Studios, is considered a bit more the successor to Fallout 1 and 2, taking place along the Rio Grande River around the states of California, Nevada (In Particular, the remains of Los Vegas), and Arizona. The game is harder, with the game's toughest enemies all spawned on the map from the start, of particular annoyance are the new flies, made to be fast, pack roaming, and poisonous. One such pathway is immediately next to the starting town of the game and WILL mean death, and that's without getting into another mission that is VERY close, a quarry full of the game's titular beasts, Deathclaws, with one more roaming along rocky outskirts, and very close to THAT is Black Mountain, an area full of Super Mutants, and an open area hiding nearby that will ambush you with radscorpions and bark scorpions. If you go in a different direction, a camp full of bandits with guns, who will shoot you, a lot. Go a different direction, ambushes by people with guns, and depending how you react to the world, more ambushes by the game's two armed military (Based on how you treat them). Someone raised on easier games will get frustrated with how easy it is to get killed, and your character is less superhuman this time. You level up to 30 in the base game as opposed to 20 in Fallout 3, but you only get half as many traits to apply, so while you'll gain much more across the various stats you ultimately are given less advantages, especially when you come across many smaller, more heavily armored or much quicker, targets, deathclaws being the worse. But it's fun, and it's again a huge world to explore. The nuclear attack is almost non-existent in this world, with the story explained through the main game what happened during the time of the attack, but the game is no less relentless for it. If that's not hard enough, you can open up a secondary mode to the game which requires the character to eat, drink and sleep to survive, along with medical stim-packs healing on a timer instead of instantly, and damaged limbs requiring a doctor's kit or an actual doctor to fix instead of doing so with your stim-pack. Even though you can max out your weapon skills, and then take that same weapon and augment it to aim better, shoot faster or hold more bullets or any other variations, the many enemies will still cause problems and difficulties through the whole game, minus any non-militarized human. There's also some creativity to the missions, which go from gambling to killing to restoring a World War 2 Bomber to attacking a bottle camp factory to protecting the president of the United States/NCR Army (or killing). Another thing to keep in mind however is that this is a very buggy game in it's raw form, crashes plenty and creatures falling easily through the ground in many many instances, and NPC's sometimes turning violent with no provocation (This happened with me while working for the NCR, and resulted in difficulty finishing the storyline as an NCR Loyal as planned). But it's still a good game, and if you're playing the PC version I'd look into some stability mods and the DLC, which expands your character's storyline from Non-Existent to "Holy crap, who exactly AM I?", as the base game has you as a simple nobody affected by circumstance.
On the whole, both games are worth playing. Which one a person likes more will fall to choice, in my case it's Fallout 3 with a very slight edge. The depression destroyed world and the intrigue to learning what all happened in the world itself, rumors of doomsday and learning what happened on the fateful day by surviving ghouls is an amazing thing to do, and the game itself is easier to play and enjoy without the fear of constant crashes, glitches, breaks, and it gives you a long and complex story with many many places to visit and see from the underground city of outcasts, the shore-struck battleship turned city, the presence of the Brotherhood of Steel in the background, a potential alien abduction, and a war between a crazy lady who commands ants and a crazy tinkerer who commands robots, who make a small town their battleground in attempt for supremacy over the other. It's simply a very diverse and silly world, which is fun to mess around with an explore. This isn't meant to denote Fallout: New Vegas in any way, but while there's still the absurd and silly in the world it's still a more serious title for those wanting a rough and tough experience, to give you the idea of living in a war torn desert amids the ruins and chaos of a long gone nuclear bombardment.