Well there was this (written by me) a year and a half ago: http://forums.tigsource.com/index.php?topic=9318.0
I should update that a bit. Since then quite a bit has changed in both engines. Box2D switched from using a sweep and prune broadphase to an AABB tree much like the one in Bullet. I'm just about ready to release Chipmunk 6 which has support for 1D sort and sweep, a Bullet-like AABB tree with really good temporal coherence, and the old spatial hashing. I added sleeping to Chipmunk about a year ago now, and improved upon it again in Chipmunk 6.
I've also made Chipmunk a *lot* faster in the past year, and Chipmunk 6 should make it even faster. https://spreadsheets.google.com/ccc?key ... y=COOb4o8N