It's interesting...I like Forza 3, it's a great game. It is certainly more consistent visually than GT5. However, I see GT snaring me for a greater amount of time. The career set-up hasn't changed an awful lot, yet it still feels more open than Forza's, and in spite of the ugliness of many of the standard cars, there's so much collecting to be done.
Moving on, there's the handling, which simply feels just 'right' - more so than any other realistic driving / racing game you'll find on console - and the visuals which, in spite of the moments of ugliness ultimately look considerably more realistic than Forza's bright, exaggerated colour and lighting scheme. GT's visuals at their best, all wonderfully organic lighting, realistic hues and impeccably constructed dashboards, are apology enough for the last-gen elements you'll experience during your time with the game.
Ultimately, they're both great games. I suspect GT5 will end up consuming considerably more of my time than did Forza, but really...there's masses of fun to be had in both. Forget about the reviews consensus being a few meaningless percentage points higher for one or the other (I believe a Killzone 2 - Halo Reach comparison has already been made) and simply enjoy whichever you access to / prefer.