I can't tell you about guitarwar because I never joined it, although I'm still thinking about it. :) However, there's another guitar competition site called
http://www.guitarbattle.com which I participate at regularly. My username there is Plasmaball. I can't tell you which is better, but I'd assume that guitarwar is because it's a pay site. You probably would be better off there if you're looking to improve your technique, skills, or whatever. However, if you're just looking for a good battle, guitarbattle is pretty fun. The calibre of talent on these sites is phenomenal. You're against the 'creme de la crop'. It's certainly a fun way to meet new musicians, exchange ideas, and showcase your talent. :)