Hands down, BuddyPress is much easier on the eyes than Elgg. There’s more activity on the homepage and whatever your WP site’s theme looks like, your BP installation will just look as pretty. Elgg, on the other hand, is the ugly duckling that just won’t grow up to be the graceful swan. The whole mods/views system it’s based on is a freakin nightmare. Most of the files that control the look are hooked up to one another, so if you’re changing the layout for the homepage, you’re changing the layout out for several other pages in the process. Now, is that good or bad? To me, it’s worse than bad. It’s just atrocious. And while Elgg’s css file gets pretty granular, it’s sooooo long that you’ll have to print it out and actually see what’s going on. Personally, I feel that BP has more control over this issue than Elgg will ever have. Also, if you’re on WP right now, you can port over your theme to BP… good luck doing that with Elgg. That said, if you don’t mind the sort of bland layout of Elgg (the default homepage isn’t too bad, but the profile pages are pretty heavy on empty space… and not in a minimalist sort of way), this probably won’t be a huge issue for you.