2 of my roommates last year had discrete MacBook pros, both 15". They called them MacBook blankets(not joking, they said it all the time) because no matter how cold it was, if you were using it you didn't need a blanket. They got HOT, like really, really HOT. One of theirs fell out of their backpack onto marble tile because he didn't zip it (in his defense; he wasn't sober, nor drunk...so I guess he forgot to zip it, and we didn't notice fast enough to tell him...), the case got JACKED up BAD, the SuperDrive continually tried ejecting when no disk was inserted, over all it just wasn't pretty. My 13" pro fell ~3.5' onto hardwood and landed on the front left corner; you can't notice there's a bend/dent unless you're specifically looking for it. And my 2010 7,1(2.4GHz C2D, 4GB RAM) ran circles around them; not just a little faster, they got spanked. One even did a clean install of 10.6; it was still like the Lakers playing a college team. Not to mention battery life. Both of theirs were high end, loaded BTOs; mine was pretty much a base 13" with a C2D, and having a dual core iX processor would have only made the difference more pronounced.
I know this is just my experience, but IMO they aren't comparable. the discrete MacBook pro was a beast when it came out, but its not even in the same ballpark as the unibody.