Fuck it, I'll bite. Undoubtedly the biggest factor for a racing sim is physics. Everything else contributes to the feel and immersion but physics and driving model trump all. GT5 still beats Forza, although the progress Forza has made game to game makes me think this might not be so cut and dry next iteration.
Graphics, you can nit pick IQ things like aliasing, tearing, shadows, etc but the entire world of GT5 has the better lighting engine by leaps and bounds. So it doesn't matter if you are in photo mode, driving a premium car or sad face standard car, every second of your experience looks better in GT5. In real time, in replays, in photos. Lighting is king and GT5 has it mastered.
Sounds, Forza wins by a long shot. GT is still a laughing stock when it comes to sound. It is an embarrassment to any fan of the series and is absolutely inexcusable for a dev as prestigious and wealthy as PD to put out such awful sounds FX in their games over and over. This does break some of the immersion for me in GT5. Luckily the above two "wins" out weigh the lack luster sound. I am a huge critic of GT sound and I find defense of it silly.
GT5 is better looking and moreover, better driving. Forza owns in sound and gameplay features. GT5 is the better racing sim, Forza is the better racing video game. Whatever is more important to you will determine which one is the better game. I need the feel of the cars and accurate tracks so I stil prefer GT over Forza on consoles. I buy to drive cars on my favorite tracks, not play MSPaint.
Is that so hard? I'm sorry but if you still want to come in hear and tout Forza graphics and physics, you come off like a dolt. It is still cut and dry. Forza is great in some areas an GT5 is shit in several but some aspects of this debate are so silly. Great, your opinion is , "GT5 looks like shit" awesome you are welcome to have that. But having no leg to stand on makes you look like a butthurt fanboy, and that is ok. We all have games that bring out the irrational hate. I think Super Metroid, Mario Galaxy, and all but one Zelda game are overrated nostalgia laden garbage, but I am never going to try to convince anyone otherwise because it is personal opinion alone. I cannot objectively demonstrate why those games blow. I can only say I don't like those games becuse of me, not because of the games. I see post after post of GT5 gfx superiority but you just keep saying it looks like shit. You can prefer Forza grahics but an objective side by side always puts GT5 in front for gfx. Continuing to bang on like that doesn't mean you can't have an opinion, it just makes your opinion kind of shit. I am all for being proven wrong, but I have seen and experienced the side by side evidence for years now and nothing new on the Forza side had made their game look better than it did when this thread first fired up.
GT6 could be PES2008 to Forza's FIFA08. The changing of the gaurd, but we're not there yet.