I hope this opinion will help you little bit.
I own 2010 ozone C4, I am on search for perfect kite since 2009 and every year I spend lot of money to buy new kites. My goal was to find perfect boosting machine (which I find in 2011 Ozone edge. This was and still is best boosting kite I have flown. If you looking for smooth hights and hangtime. Yet if you looking for agressive feeling (kind of like your ribs are pulled away from harness, you need to choose more C shape kite). So as my riding style changed I was looking for perfect kite again, and realize that for great jumping (not so great hangtime) little bit of unhooked and waves C kite will be best. Unfortunately I start kiting late, I never liked much 5 line kites (I was spoiled on 4 line and hybrid kites) so I was looking for 4 line C shape kite hybrid. Best boost and hangtime is by my opinion at Airush xVarial and Ozone C4. I have not tried all c-hybrid kites on market but I tried few).
So to go back to your question I ended up selling whole quiver of xVarial and buy 2 year old quiver of C4. They are awesome kites. Only thing I wish for is they are more durable (canopy) and they hang longer .
Last week I had a chance to try my friend new 2013GTS 12 and compare it to my 2010 C4 12,
the kite has lot of similarities, yet I would choose my C4 over GTS2, this is strictly mi opinion, and it may be mainly because I am use to this kite.
here is what GOOD I can say about GTS2,
- it boost
- it has more low end power then C4
- light bar pressure
- pull away bar safety syste
what I like about C4
- I believe it boost better and slightly better hang time
- both are easy relaunch (I felt easier with mine but it may by because I am so familiar with it)
- I love when middle lines spread close to depover (for easier and faster self-landing) ozone has that
- bar is thicker which I preferrer
- no pulleys, just bridles (simple and effective)
I am sure you will be happy with either, the best thing will be for you to test both back to back. I never flown 2013 C4 and I plan to buy new once they will come up with new bar, Ozone bar is great, it would be better if is has pull away safety and stopper ball for dead man