I didn't used both but gave try to bespin when it appeared (it evolved to cloud9 as i know) and liked. it's an advantage to have a universal gui, to not ignore users of other platforms.
btw, i put some effort to build an online ide 4 years ago. i stopped maintaining it because my dream tool was evolved to firebug lite. anyway, my died ide can be seen at vimeo.com/azer for some web nostalgia
Just a small correction, Bespin evolved into Ace (which Akshell uses as their code editor), which was created by the guys that created Cloud9 (Ajax.org). Cloud9 is the actual IDE that uses Ace as its code editor.
btw, i put some effort to build an online ide 4 years ago. i stopped maintaining it because my dream tool was evolved to firebug lite. anyway, my died ide can be seen at vimeo.com/azer for some web nostalgia