The perf is generally comparable (each has optimizations the other does not, although gambit is a bit faster in the general case), but Chicken has really fast call/cc. Also, Gambit has a very small library collection, especially compared to Chicken's, which is among the best of all the schemes, Library-wise. Neither supports native threading, and Chicken isn't fully re-entrant, which can lead to some problems integrating some libraries. Both have excellent FFI, but Chicken's is a bit better, IMHO.