neither. the AF120 are pretty much the same as the the ones corsair gives you with the 120mm water coolers. you can "upgrade" to the SP120s but there will be significantly more noise. in either case it will be a waste of money.
a much better upgrade would be these:
the COUGAR CF-V12H fans are quieter than the AF120 but still match the air movement of the SP120. and the best part is that you can get them for $5 a pop after rebate now.
EDIT: tominsac below me has pretty much just confirmed what i said.