hi everyone

actually, i decide to change the crank set and folk of my mtb

but i'm really confused now.

well, my frame is cannondale optimo si (06 model)

my friend suggest me to buy si crank. Yesterday i phone to the bike shop and asked how much of the si crank. They said that it costs around500$USD.

well....can anyone tell me...if the si crank worth of this $$?
some of my friends said..don't buy si , kcnc/raceface is better..

..can anyone give me some opinion?
money is not the problem...i just care about does it worthy...