This is actualy a realy good question. Ok, where to begin, I'll start off by saying both the sv650 and the Gix 6 make excellent track bikes. A qwik note about the differences, the Gix is an inline 4 with approx 100-105hp stock. The sv650 is a vtwin with approx 70hp. Big difference in hp but yet the sv650 and the right rider can keep up with just about anything out there. The only place where the sv is lacking is in the straits, but even then if you get a good drive coming out of the turn and onto the strait, it'll take a little longer for someone to pass you. BUT, all straits eventually come to an end and you need to turn, this is where the sv comes out on top. The inlines having more hp carry so much speed down the strait, that they have to be hard on the brakes to set up to tip it into the turn. While they are braking, someone on an sv is still on the gas because of the less hp and mass amounts of engine brake...you can wait longer to brake and ultimately carry more corner speed. To put it in perspective, there are guys in the blue group at summit turnin 1:27 laps at Summit Point on Gix 1K's while others are running 1:28-29's on SV's...I think the fastest time for an SV are Summit is in the 1:25's...but thats just insane. Theres also less chance of the rear breaking loose when powering out of a turn, not that it won't or can't happen, but again the lack of hp, and you get the idea. Both bikes are easy to ride, both have great "flick ability" and are a ton of fun. At the time when I starting doing track days I was using my Gix 750..which ultimately led me to be intimate with turn 1 at Summit...after which I looked into it and the sv seemed more practical for me and I don't regret it one bit...it really is an awesome bike.
ok thats my .02...Hope this helps