Hard to say what the "best" bike would be....I would think the main reason to stay away from the 2-smokes would be maintenance.

An SV650 would be ideal in the following aspects
- reliability
- available spares
- easy on tires
- eligible for a ton of classes
But in most classes it wouldn't be terribly competitive in the hands of an inexperienced racer (ie, someone who is just getting started or is just trying it out)

A 2 or 3 year old 600 might be better from the "being competitive" standpoint.