Which do you prefer and why?

For me, if Heroes of Might and Magic 1-2-3-4 is a 10, then King's Bounty Legend\Armored Princess is a 9.

Honestly the only thing keeping King's Bounty from a 10 is the replayability. I played Legend three times and I'm kinda done with it. I don't think I'll ever play it for a forth time.

The thing that I elevates both series above most other turn-based games is the deterministic nature of the gameplay. Sure the creatures do have a damage range, which involves a certain amount of RNG, but whereas shitty games like Hearthstone have outrageous random effects such as your spell does no damage or you win the game, good games have smaller damage ranges allowing for actual tactical thinking. In this case, Heroes and King's Bounty the RNG is almost non-existent, especially since you are given multiple tools to offset RNG even further.

In a way I like King's Bounty's simplicity, in the sense that you have only one hero and don't have to bother with building castles and expanding. I never had the patience to finish more than 2-3 Heroes campaigns because it tends to get repetitive after a while.

HoMM3 between those two, but in the genre itself, Disciples 2.

Heroes 1-4 are, in all honesty, the better games, but I kinda agree with blotunga, I also enjoy the simplicity of King's Bounty, and the lack of pressure. There are no AI factions going on the offensive, no time constraints, I can do the quests and explore the world at my own pace to my heart's content.

So it kinda depends on what I'm in the mood for, but King's Bounty is more friendly to just pick up and play, also better for playing in short bursts.

And Matewis is right about Disciples 2. That is just the best ever (but the first one is great too).

To me, I like the overall gameplay and approach of King's Bounty but I enjoy both. HOMM5 was my first game in this particular strategy genre but King's Bounty kinda checked off a few more boxes for me in terms of what I usually go for in strategy gaming.

Both are great. As highlights from both series, I'd put Heroes III and V, and King's Bounty Legend and Darkside

As for which I prefer, it's difficult to say. Heroes can become pretty monotonous after a while, especially once you complete the campaigns, but for the first couple hundred hours, it's a blast.

The potential to try out different builds and unit comps in King's Bounty is very high. You have 3 classes and in every run can choose different units you use and plan synergies between them as you are not bound by a town.

Love them both.

And as mentioned, definitely don't forget about Disciples. As far as setting, lore, art style and story building goes, nothing beats that (as long as you don't mind a very depressing setting that is).

HOMM series definitively. What I want in those kind of games are good 4x sort of games Good strategic games. The only similarities between King's Bounty and HOMM series are the series paleorigins, the graphics and the combat system.

I like King's Bounty, but it get old easily. It feels to me too linear. Lacks enough replayability. I'd like they were less restrictive, more open. i even feel it more deterministic than HOMM. Probably they are smilar in that regard but the nature of the games make that HOMM ends up being more variable and replayable.
They are good games though. Recomendable indeed. And pretty polished.

neighter
i would like a new real time heroes ,taking turns is so meh thats the worse by far , i would even delete all copies so nobody has to endure that boredom

Yeah unfortunately if you know how to play the game then HoMM3's base campaign, and most of Armageddon's Blade is pretty easy, and the feeling of repetitiveness kicks in fairly quickly. Shadow of Death on the other hand is a different animal, and to me at least didn't have that repetitiveness problem, perhaps because of the difficulty. Iirc the 'Retrieving the Cowl' level took me the greater part of a weekend to finally beat

I would argue that Kings Bounty offers more replay ability than HOMM, although I didn't play the later that much.

As someone who recently finished the second playthrough on Kings Bounty Armored Princess/CrossWorlds, I would say it's one of the best in series but people who like reading all the text and embrace themselves on the story, maybe prefer The Legend.

TBH I don't like RPG's, don't know how to play them, don't get the fun in play them and don't know what RPG even means...
That said, playing King's Bounty on higher dificulties it's the way to go, otherwise the game is too boring and repetitive. Putting some artificial dificulty, like no losses, no offensive spells and no rage use makes you know the intrinsicate details of the fights and units. Once you get to understand that the less units you use the easier the game is, beating "impossible" or "lethal" enemies with no losses is not that dificult with the right units and equip combo.
The replay ability comes from using different hero and/or styles, not the story (as if it was not boring enough). Playing with a Mage hero mosty focused on attacking spells (the easier way) is way different from playing with Warrior hero.

"""""Spoilers""""""
On The Legend, as Mage you can beat the game using nothing more than Black dragons or a mix of Black/Red Dragons once you get them. Before that Royal Snakes are the best friend you can have.
If you're playing Armored Princess/Crossworlds, give Black Knights a try using Moro Dark as companion. Once you get Moro Dark, kill as many undead as possible to get the Darkness Power to 0, you'll receive a item that completly breaks the game...

I wish I knew how to play the HOMM games... I started playing the first game but it just felt oddly complicated... I mean, how am I supposed to know some different kinds of monsters don't get along together in the same "team" and they give you a penalty or something if they are in the same party?

I think I finished the first "level" in the campaign, but I kinda lost interest in the second "level" where one enemy attacks one of my castles and I just don't have enough manpower to defend it. Screw it.

I read some HOMM FAQ but it made my head spinning, making it all look so much more complicated, LOL...

Maybe some day...

Apart from defending not working like you said, i.e. you have to constantly explore and expand (not feverishly necessarily, unless it's Shadow of Death), there's one other perhaps even more important thing that bears mentioning for anyone struggling with the game: Don't have your main army waste time running back to towns to gather troops! Have dedicated 'ferrying' heroes do that for you.

Equally detest the gameplay of both.