I am not sure exactly what 18:1 SFI means but I know it is a measurment of balance related to rpm. A SFI balancer will work faster than your motor can spin.
SFI approved mean's it is approved for the track. Basically the balancer is shatter proof if it's an SFI balancer. Most track's or sanctioned racing division's will require you to run an SFI approved balancer if your running faster then a set time in the 1/4 for safety reasons. The same goes for Flywheels, Bellhousing's, racing suit's, helmets etc. etc. etc.
Stock style (non SFI) balancers will work up to 6500 rpm. Unless you're spinning past that or like the other guy, need to because of track rules, then just get a stock replacement one. Oh yeah, they're a lot cheaper than SFI balancers as well.