I was only aware of PortableApps, until I bought a new SanDisk USB Flash Drive and found that it came pre-loaded with U3. Then I was a bit confused because it seemed to be covering much the same territory as PortableApps. (Should I use one or the other or both?)
I think it would be really useful to have a link from the PortableApps home page to a page that compares and contrasts U3 andPortableApps.