I'm a windows / LR user, if I went Mac there is no way that I would use Aperture due to this issue. There is no way that I would want my image files stored in the database file along with the edits as Aperture dose. Having everything all inside one file would worry me no end, what happens if the file gets corrupted? You loose everything, including the original images. At this point you hope that your backup is ok. It's not? Oh well guess you better get out there and start re-shooting those pictures, but you can't re-shoot never mind.
With LR I have all of my image files stored separately on the drive and all the edits stored in the catalogue. So the catalogue gets corrupted, run your backup. Oh dear the backup is corrupted too, well at least at this point I still have my original image files (which as they are sitting on the disk should have been backed up individually too) to use to re-edit from.
Another advantage to having all of the files sitting on the hard drive is that if I want to use a different application to edit a few files then that's easy too. the file is just sitting there ready. The only thing that you should remember with LR is that if you do not want to have to have LR try to find "Lost" images is to only move the files from within the application. Use external drives? That's fine too, LR will still show you the preview image, just have to connect the drive to edit it though. Multiple drives in one computer also work, I guess that with Aperture you are limited to having a single catalogue on a single drive. In LR you can have the same image in multiple catalogues too if you want to, with different edits too, or you can import the existing edits to a new catalogue if you need to.
As you can see I have lots of good reasons to use a fully referenced system, over holding the image data within the catalogue files along with the edits. The difference in the catalogue file sizes will also be significant, and I'm sure that will also affect system performance, after all disk reads are pretty slow, imagine having to read a Terabyte catalogue file in order to open the application. I have only had a play with Aperture in an Apple Store, and given it was running on the latest hardware the performance did not seem great compared to LR4.1 on my 4 year old Dell laptop.
Alan