I went the KMC route after a bout of interference put me into a wall and snapped the original chassis in half.........
Certainly transformed the car indoors - I race on a polished wooden floor and with the kit chassis I used to get understeer one way, oversteer the other. Now I know I have equal handling side to side which means no nasty surprises when I give it the beans - at least - it will when I get my motor and cells sorted.......
I've only raced once outside since the conversion and it was very easy to drive - even on my indoor setup 'cos I ran out of time to swap over. I never ran the kit setup outside so couldn't comment on a comparison.
So for me it's KMC - cost me $178 (£92) including shipping and the extra rear layshaft kit you have to buy to get the drive cup.