100cpi is (100 cells per inch) less filtration, so more free-flowing, likely to light up the engine management light on the dashboard and emissions could fail MOT, also noisier, may have implications on fueling/air mixture confusing ECU - but I couldnt say for sure .
200cpi - the more accepted one - will not affect Engine management - more expensive, but quieter, usually better quality manufacturers make these.
if you are going for all out noise from the exhaust then 100cpi - otherwise if you can afford 200cpi
all the best
David