Giants has many different styles of gameplay depending on which race you're playing as.

I have both on disc. I enjoyed Hostile Waters more, its sort of like a strategy-lite version of the PC Battlezone game. Giants while humorous is rather unpolished.

also, check out sacrifice if you haven't already, because you basically just described it in the latter half of that paragraph.
a note: in hostile waters, you control squads by pausing the game and going in to a war room to issue orders, you can't issue orders while you are controlling a unit, unlike in sacrifice. amazing game though.
i haven't noticed too much boring running around in giants.
to be honest, i would have to recommend that you bite the extra money and get both. they are awesome games and you will get hours of enjoyment out of each.

Giants.

I will second ELPixelllustre.
I like the gameplay style in Giants more than an overhead view of the action. That probably stems from my need to get my hands ddirty when carrying out a task rather than delegating it to someone else (i.e the damned AI) who will not perform the task properly...

Thanks for all your opinions.
I remembered that I had this dilemma a little while ago when I had to decide between Beyond Good and Evil and Psychonauts. I'd read so much about them and seen so many opnions I ended up fussing over all these little things that might make one game better than the other (like deciding between apples and oranges in real life).
When I realised I was wasting my time and would be able to buy the other one later when I had money again, I decided I would flip a coin and stick with that.
I called heads for Beyond Good and Evil. That's what it landed on, so I bought it and found it to be an absolutely fantastic game that I spent a lot of time with. Then later on I bought Psychonauts and had a blast with that too.
So today I flipped a coin and called heads on Giants. That's what it landed on, so that's what I'm getting. They're both good games it seems, I just needed to randomly decide what I wanted.

I'm too late, but I'd really recommend Hostile Waters. Giants is great too.
They both have slightly similar presentation, with great OTT characters and a slightly surreal feel (though giants is more surreal).
A few things people mentioned about HW aren't correct... you can issue orders while in the action (though its probably easier to do it from the paused overhead map if you want to issue complex orders. )
I loved giants too (except the kabuto levels) but i just feel HW is really unappreciated :-(

So is HW just a RTS game? Where you command your units and build building structures?

Its an RTS / FPS hybrid.
(Except much of the RTS is not R, and the FPS is TPVC)
You can play it almost totally as an action shooter, or you can play it almost totally as a strategy game. Or you can mix it up.
It doesn't have a freeroaming camera, your camera is behind the units, and at any time you can jump into the units are control them. (driving/flying/side strafing, etc..).
You have 2 ways to issue orders to the other units:
First you can pop into the strategic map, a simplified 3d rendering of the whole battlefield, and issue them orders with the mouse. The game is paused at this point.
Second you can issue realtime "verbal orders" while still in your vehicle. This works in a Subject-Verb-Object way. Eg: "Unit3 -> Attack -> My target". "Unit2 -> Pickup resource -> My target, -> take to recycle unit".
I tended to mix up both playstyles, though pausing the game and issuing detailed orders is easier in tricky situations.
Generally the AI is pretty good and you can mostly leave them to themselves. (except air units near AA)
You don't build buildings (except maybe turrets). You do build units. You do gather resources. The enemy is more like a traditional RTS, in that it has factories, oil derricks and energy stores.
Note: It has no multiplayer. This is a shame as i'd have liked to have tried the multiplayer with only the realtime commands options.