I have the FK452s, and have driven a car with the SPTs.
I can tell you that you cannot go wrong with either tire. I bought the Falken because I've used them before, and they stretch decently too for my needs. I've also heard that the SPT is very good in the rain, but my 452s performed VERY well in some hard-braking I put them through in that massive rainfall we had about 2 weeks ago.
I believe the price difference is very minimal between the two, also. Take your pick!
Imola Red / Imola Red ///M5
E60 SSK, Dice iPod, Illuminated ZHP knob, GT Custom Exhaust, BCR Coilovers
'01 330Ci Titanium Silver on Coral Red
'96 328i Jet Black on Black