Either would be fine, I doubt it'd make a difference.
The only reason to choose one over the other is if one is crippled with a 64-bit memory interface. Do a little research on those specific cards because I believe both come in either 64-bit or 128-bit memory busses. A 128-bit memory bus would be far superior to a 64-bit bus.
Keep in mind, that's not onboard RAM, like 128mb or 64mb... the memory bus is an interface, and it will be specified in 'bits'.