Blackberry OS is pretty rock solid, but I dont knw of alot of freeware for it.
N73 is more multimedia oriented, with 3.2mp cam as opposed to 1.3mp on BB.
BB in my opinion is more executive style phone with a better build quality. I've played with both and BB is really thin compared to N73 and feels 'just the right size' in your hands.
N73 is a little bulkier, but thats because of the camera and other multimedia features. Thats not to say there is nothing on BB, its jst got a lower resolution camera. But the pearl's interface is WAY better then N73, thanks to the pearl itself.