If you have limited time with engines, I'd say go with the Evo. STs are generally good engines, but they do seem to be a bit harder to tune right, espeically when new. If you go with the ST, and you have running problems, try leaning it out. Twice I've "fixed" STs at my field by just leaning them out. For whatever reason, these engines didn't act like others when run rich, they just got funky instead of running in a slobbering rich 4-stroke mode.