how big is your library?
I ended up with Aperture because I desperately needed to merge three different iPhoto libraries. I don't have much of a problem with its feature set, and PhotoStream integration is huge, but I haven't tried Lightroom so I don't know what I am missing.
I do have do a HUGE complaint: performance (on my 2009 MacPro with 8GB memory), working in JPEG, is just awful. Beachball city. I do have a very large library, but still.
Also, no easy (or indeed one which I have gotten to work) way to watermark photos.
If a hugely improved Aperture X doesn't appear by the end of the year, I am jumping to Lightroom.
In the meantime I am going to stick a fast SSD in the MacPro figuring it the single most effective way to up performance.
Did the preview generation setting help?