I use Aperture, and have since v2. I've not yet tried Lightroom, though I am tempted.
On the positive side...
Aperture works very well. I can quickly review photos, rate them, and make simple edits. When I make comparisons with DPP, I find the Canon s/w wanting. I still get a better image from Aperture.
But what good is a photo if it isn't printed? As with iPhoto, Aperture has a book creation ability that is much more flexible than iPhoto. Additionally, you can use templates from selected outside vendors. I've made many books from Apple, including a few through Aperture. In Aperture, you can edit existing layouts and create your own page layouts if you wish. While I don't make new pages from scratch, I occasionally make modifications to standard templates.
On the negative side...
Thank goodness I'm a computer geek by nature and profession. Twice now I've had Aperture $* me over on "permissions." OS X is Unix. Directories and files have permissions for access, granting or denying access depending on who you are. Twice I've tried to recover libraries from backups. The first involved 12,000 photos. The second incident involved 3,000 photos and just ended this morning. Something on my iMac (OS X itself, Aperture, or Time Machine) added some new permissions to everything in my library. When I tried to restore a library from backup, it said I didn't have permission to do so, and wiped my library.
Two. $*ing. Times. No joke. The original raw images were fine; what was wiped was all of the edits, the light corrections, the straightenings, and the occasional retouch. This last time started two nights ago. One command line entry (that looks like cartoon swearing) and a day and a half later, and all is once again fine. There was no data loss.
Googling the problem and working with Apple the first time showed me this is an obscure problem. Apple didn't know how to fix it. I was and remain on my own, which is fine. Knowing how I am with s/w, I could have just as easily encountered weirdness with Lightroom. At work they call me when they want someone to break things. I does good at breaking s/w. 
Conclusion
I'm sticking with Aperture for now, though I'm going to still try Lightroom. Check online reviews, and you'll see that the comparison between the two is probably similar to the comparison between Canon and Nikon. Take the time to try both Aperture and Lightroom on the 30 day trials. Process images through both of those, and do DPP too. If you like DPP best, then just stick with that.
Recommendation
If you go with Aperture, go with at least 4GB. My iMac has 2GB, and a few days ago (before the aforementioned "issue" occurred), I ordered an upgrade. While my MacBook could also benefit from an upgrade to 4GB, I don't use it enough for photo processing to upgrade it. 2GB will work, but slowly.
Good luck!