It's best to try a bunch of settings/clients out first. I've seen slightly higher numbers with Reaper, but I could never get it working 100% (connection issues), so I'm using CGMiner by default with a pair of 7850's. You might as well make a dummy plug as you can get a 50 pack of resistors from eBay for $0.99, just in case (
for context)
EDIT:
GUIMiner is just a frontend for Reaper/CGMiner, it's not needed, it does help with trying different settings quickly and not resorting to .bat files. I like just using .bat files, but GUIMiner is great for trial and error before nailing down final stable settings in bat format.