i think it depends on how confident you are. if you are pretty confident overall on your bike then i don't think it really matters which track you do as long as you take it fairly easy. my first was at lydden, which is short and easy to learn. my second was at silverstone gp, which is pretty much the oppoiste being long and very fast (which my 33bhp at the time struggled with a little). i loved both despite an off at lydden and went back this october (this time with full power

) and still had great fun and i can't wait to do silverstone gp again, but with full power

.
where ever you go for your first one make use of the free instructors and ask one of them if they'll go out on track with you. i did this at silverstone and it improved me a huge amount and by the end of the day was getting my knee down regularly