i went from spd-sl, to speedplay and then back to spd-sl just because the speedplay pedals have a lot of slop in them. they were hard to dial in. i mean, if you plan on getting a pro fit, id say go for it, but if you're just playing the guessing game of what feels good, i say stick with the spd-sl cleats.

also, i dont know how true it is for other people, but in my experience the little extra surface area of contact that the spd-sl pedals give over the speedplay pedals eliminate hot spots on the foot and just make for an all around smoother transfer of power. again, i am not basing this on any studies or anything, purely conjecture.