Thanks so much for the advice, guys! I'll just have to ride both and see.
My current bike (a Jamis Quest) has all 105-level components, and they're very nice but I would really like to step up to the next level, which is why I'm looking at the 5.2 instead of one of the "lesser" Madones (plus the $500 discount that a bunch of you pointed out helps make the 5.2 a little more manageable!). It would certainly be tempting to get the Tarmac and and some rad wheels for the same price as the Madone.