Well I can tell you as I have both SQ Burns and ISIS Burns.
The SQs are damn good, strong, light, pimp
The ISISs are also damn good, strong, light, pimp BUT also stiffer. They just feel slightly stiffer that the SQs.
Also if your isis bolt comes loose on a ride, you wont round your cranks off :turned:
The isis BBs tend to be a little more expensive but its worth it :D I've had my raceface SRX (£30) for two years now with no problems.
Sam