One of the main differences is that activeCollab is a self-hosted solution so some of its functionality will depend on your server setup. Basecamp and Goplan are both hosted by 37signals and Webreakstuff, respectively.
Goplan is a bit cheaper than Basecamp and the starter package on Goplan gives you 500MB of space as opposed to 400MB of space on Basecamp (although Basecamp has the option to host your files on your own server and this removes the space limitation – if you go with this option make sure your files are properly secured from unwanted viewers). Notwithstanding the price, I am going to stick with Basecamp, particularly with Highrise in the works.
(Source: Paul Stamatiou)