Which should i go for, if i'm honust i don't know too much about cars.

I've been told by a few people that if i convert my duel mass flywheel on my 1.8 TDCI focus i wont tell the difference and it will be fine, but i can't help thinking that if there is a duel mass on the car in the first place, and the part costs this much - why is it there if it isnt neccessery?