The former looks so much better than the latter that it makes me think the former picture cannot be in game! :/

Ground textures might be a sore spot for Forza 3. The ones on the cliff look damn good though.

Because of the location of the Forza 3's map. GT5 map has to be perfectly round because of the place where they race.

Though in a racing game, its the easiest thing to cut. Though I wonder if they've got the tessellation engine working on the terrain? Its a hard thing to get right, but if they do it it can really increase the polygon density quite cheaply. Ensemble used in on Halo Wars' Terrain as well.

Because everyone knows, a game can't be any good unless this guy's friends know about it.

Nobody really knows how GT5 will stack up since it wasn't around. But based on the direction the franchises have been heading respectively these are my opinions on the two.

Better physics = Forza. GT has played nearly identically since GT1, which is to say not nearly as well as an evolving franchise should. riding walls around tracks, blasting through sand pits without losing speed, bouncing off other cars with no ill-effects, etc. GT has nothing on Forza's robust physics setup.

Vehicle tuning/customization = Forza. From aftermarket parts, including replacement body parts, to the extremely time consuming paint editor, you can take any car in the game and make it your own. The online community surrounding just this aspect of the game is still very active in FM2. You can create a paint scheme, send it to your friends, and all race on a team with the same schemes, you can recreate basically any image. It's limited only to imagination and skill.

Visuals = Car models are a toss up until I see GT5's damage models. Right now I still have to say Polyphony get the lighting just perfect which goes a long way to making the cars themselves look more at home in thier environments, but from the FM3 videos that came from E3 it seems Turn10 has figured it out. As for background visuals Forza trounces GT (compared to GT5P by a long shot, and I haven't actually seen any in game GT5 yet) I found GT5P backgounds to be almost void of character. Are they ugly? No. But they look bland and almost like everything was just built. In contrast even Forza 2 had a much more 'used' look to everything in the tracks.

Sound Effects = Forza. Polyphony needs to start using a dyno to record the cars under load, not sitting in a parking lot revving. When I played with a Z06 in GT5P it sounded nothing like the snorty growly car I've been droolingover since it came on the market, it sounded much more like a 1996 Ford Escort. In Forza that same car grunts and growls under acceleration like it should. Do all the modification and engine part upgrades to a Hemi in Forza and compare it to a GT game muscle car and the difference is laughable.

Online community = Forza. Not to start up the Live vs PSN debate, but GT5P online was terrible. Everyone knows there is no penalty for smashing into things and they all take full advantage of it.

Available before 2030 = Forza. Henestly, I can't even figure out why this even being contested, there is only one contender with a release date. I don't blame Yamauchi though, as long as he's 'working on the game' he has access to any car he wants t play around with, lol

Seriously GT5 will likely be another year away the way things are going. Yamauchi didn't even go to E3, nothing was shown of the game but a quick cutscene whichnobody will confirm was even in game footage. The GT franchise is played out. All fluff and bling with nothing under the hood anymore. I'd go so far as to say I consider GT more of an Arcade/Sim than a true racing sim.

why can we just give it a tie?. and everyone wins¡¡¡¡¡¡

That is a good point.  Still shots or the starting line are probably the only time to see the ground textures closely.

I did not know that Ensemble used tessellation in Halo Wars.  The terrain was nice in that game. 

Turn 10 may have used it considering how well that track and the surroundings looks.

The thing that really shocked me was that GT5 have also the NASCAR AND WRC license. GT was always a huge game for car fans but those two licenses made GT 5 even more awesome.

I am normally not that kind of person who would say GT5 will be more awesome without watching ingame footage but those two licenses are really a big deal and makes it personally for me hard to believe FORZA 3 can surpass it.

(Not that I am going to buy one of them though).

I saw that in the GT video.  That could be why it is taking so damn long.  It is like having 3 games in 1.  Perhaps Sony should have made them create 3 different games.  If they succeed in all those areas though(not a fan of Nascar to be honest), it will be the most comprehensive racing game ever.(I think, although I really have no clue)

My partially educated guess is that cars are the ideal suitor to the tessellation engine in the Xbox 360. They are quite regular shapes. Microsoft released the SDK for DX11 (Incl Tessellation) at the end of 2008/start of 2009 so they would have had it in development and available to their studios for much longer than that. I suspect that the tessellation engine took a load off the shaders which could then be applied to the cars in the form of the likely extremely complicated shader ops for the paint etc.

I haven't played Halo Wars yet, sorry!