which one is better?

HL3

Personal preference, as with any other game. Personally I prefer Fallout 3 GOTY.

Fallout 3, more post-apocalyptic environment.

Fallout NV has ironsights, fallout 3 doesn't. I like having ironsights. So I say Fallout NV although, I haven't played FO3

Fallout 3 is better

I don't know what people are going on about FNV is much better.

New Vegas

Fallout 3. But im kinda biased since i live right outside dc.

Fallout New Vegas is one of the best RPG of all time that should sit in WRPG heaven with Planescape Torment, NWN 2 Mask Of The Betrayer and Baldur's Gate 2 and i'll tell you why comparing to Fallout 3 is like comparing a swiss chocolate bar with a pile of putrid shit.
Fallout 3 has a giant but empty world: sure is bigger that NV's wasteland, but also lacks of the interesting sites that NV is full of. In NV you travel in a smaller land that don't annoy you with dozens and dozens of enemies during every step and let you survive with a non-combat build.
About builds, in F3 YOU HAVE to spend some points and perks in some combat skills as you'll encounter really a lot of enemies and a no-kill run is almost impossible to accomplish, in NV your character adapts to your playstyle and lets you play as you wish, you can avoid fights using stealth or you can talk with NPCs to complete your objectives without killing anyone. In Fallout 3 you'll be shooting a lot and sometimes you WILL encounter some enemies that will be almost invincible without the right equipment (like Deus Ex Human Revolution bosses with a stealth build). In NV this NEVER happens, even mother deathclaws can be defeated with the right moves.
Shooting is far better in NV: there is iron sight (instead of the poor zoom-in feature of F3), weapons give a better feedback and VATS has been nerfed to let the player shoot his way old style. Unlike F3, if you aim something, you will hit it, in F3 you can miss even if the target is right on your face.

Last and the most important, New Vegas IS A FUCKING RPG. There are TONS of choices, you can side with different factions that have a different way to live in the wasteland, there is not plain good or bad choices, nor right or wrong choices. There is YOUR choice, your presence in the Mojave lets you build a different world based on your actions and people will act accordingly to it.
Talking about examples, here is your tipical New Vegas quest. and here is a quantity comparison.
Plus only one F3 dlc is really interesting, while in NV at least 3 of them are good. I have 76 hrs on a single run of New Vegas, while i've completed Fallout 3 in 30 hrs.
Your choice.

Fallout: New Vegas.

Fallout new vages is more complex than fallout 3, personally i liked fallout 3 story. and New vagas hardcore mode.

I think you should get both if you can ( In the sale thats going on in getloaded )

If you already though about game to get in the bundle , you might want to play Fallout 3 goty and buy fallout new vegas ultimate pack in summer sale ( Which we all hope it will be ) , because I think its better to start playing in Fallout 3 than Fallout NV , but its my opinion , maybe you want to play first in nv :)

FN:V is more along the lines of the post apoca F1 and F2 while in F3 you still feel like in the middle of the apocalypse, much more serious than otehr F games.

Wait just a couple weeks longer for the steam summer sale and the odds are pretty darn good you'll grab BOTH for $5 each!

F3 seems better, i have played only NV.

None of it, play normal Fallouts instead ;)

Well, for me, I enjoyed FO3 more for some reason, but FNV is better, more stable, has more features, I don't know, get both if you can, if you do, get Requirem mod for FNV, it brings FO3 world into FNV so you can play both FOs at same time.

Definitely both.

New Vegas and play Hardcore Mode ;)

3 was glichyer when I played it but I think I enjoyed it more.