Even though this comparison is for the cars as they exist in the game, I find it difficult not to consider their real-life counterparts. The 458 has a true automotive soul, and I have a great appreciation for the cars of the storied Italian maker, but the McLaren is just a rush to drive and is an obvious result of its development time.
As many have done in this thread, I still have to give the title to something else in this category (ish); the Ruf CTR2 '96. More comparable to the Gallardo than either of the two MRs, this thing packs 540hp and stays surefooted through almost any set of conditions.