So much talk of bundles in this thread. Nobody really knows just how much bundles have contributed to either franchise's sales in the past.
What we do know, however, is that GT5 had sold 1541k after its first ten weeks on the market in Americas, while Forza 3 had only sold 886k. That's a difference of almost 100%. However, as of their 12th week on the market, post-holidays, GT5 has dropped to only 17k or so, while Forza 3 was still doing closer to 25k.
LIfetime sales between Forza 4 and GT5 could go either way, but I think it'll depend entirely on how much either game is bundled. That'll likely favor GT5, given it'll be around much longer before a sequel. Plus Forza 4 will be released when the 360's sales have finally start to slow down.