I hate racing sims, and neither one of these games will get play on my consoles. The disc trays will be occupied by games that don't feel like work.
Have played GT2-4 + 5 Prologue, and have lately playing Forza 2. Overall so far I do prefer GT however I dislike 5 Prologue and won't deny it. Couple minor but forgivable feelings towards Forza2. In general both great games and really comes down to which console you have. A real fight would only really happen if forza broke in the PS market or GT moved to the 360, then we would have a real battle.
EDIT: A wheel definitely is a good compliment to both games.