i had both and like others said you can't go wrong with either, the big bore shocks on the ST-RR are really the difference in price between the 2 trucks.
Mugens are known for leaking diffs, and diff pins snapping
Kyosho you have to keep a close eye on the shimming of your diffs
I think my Kyohso turns a little better and handles the rough better but they are both awesome. XRAY truck is really good too.