Check with Larry - FASTRATPORTING ( a member on here and a SUPER guy to deal with - for the mild modded engine )
As far as the comparison ....
Each has their pro's and con's .
Guess its all up to the buyer .
They say that the SIKK can not be modded as much as the ZEN , but , then again , the SIKK comes out of the box , faster and a little more powerful than the ZEN does .
And as said previously . check out the thread further down the list - talks all about the same question . Lots of GOOD info there .