356 gnr?

i just picked up a 2nd 357 blackhawk this one is a 4 5/8" barrel convertible came with both the 357 and a 9mm cly .i also have a early new model 6 1/2" barrel one. would i be better off having gary build me a cly for the 6 1/2" one my original plan before getting the 4 5/8"one ,or just rechamber the 9mm cly on the shorter one .how much velocity difference between the 2.