In terms of displaying fully rendered thumbnails or a completely loaded enlargement of a image, iPhoto 5 is faster on my PowerBook. Aperture does provide you with an image right away -- whether it's a thumbnail or enlargement -- then finishes off the loading while you view it. Thumbs often first appear as lorez, then snap into final focus a second later. Enlargements look good on first rendering, but will often continue to load information for a few seconds while you view it. I would suspect that on a G5, Aperture performance is noticeably faster.