I've been doing the same thing for a few days - comparing trial versions of both Aperture and Lightroom.
Hands down - I love the Lightroom UI.
Aperture's integration with iLife is a HUGE bonus though - so this almost equals out the last point (almost).
One GLARING shortcoming though is the lack of auto syncing folders. I'm a recent Mac user, and on the PC I used Picasa (I almost shed a tear after bying the 24" iMac when I saw not OSX version of Picasa existed). Picasa autosynced the folders you set up to watch. When I dumped photos into a a folder - I didn't have to keep track of them for importing into my DAM software - it did that all on it's own automatically.
I am profoundly saddened by the lack of this feature in both Aperture and Lightroom. I know iView and Portfolio has it, and I applaud them - but the editing tools just aren't there for these guys... I've been seduced by the lightroom UI is most likely my problem here.
Anyhow, If Picasa ever made an OSX version, both Apeture and Lightroom will be in trouble (especially since Picasa is FREE). If Picasa ties into iLife... hell - that will be the one-two punch as far as I'm concerned.
Can someone PLEASE tell Adobe to look at the auto sync feature (this is no where near the same as their watched folder feature), and integration into iLife (and iWork for that matter) would have me pre-ordering the next release!