I'm going to get two r9 290x and I was thinking of going with the kraken g10 and a corsair h55 but if I go watercooling I'll get better cooling and a bigger Overclock but it would be more expensive so should I go with the g10 or should I watercool?
if i were in your shoes...... no.
I personally don't care for current AOI water coolers. forget the possibility of leaks (which still happen), but if the pump fails... the whole thing is a brick. So i would rather stick with air cooling. worse case scenario.. fan fails. fans are cheap, you just replace it.
Now in terms of GPU cooling, it would be a LOT of money for little to no increase in performance, especially with beast air coolers. the Sapphire Tri-X cooler 290X seems to be pretty beast. spend the $125+ on kraken and H55... i bet you see no difference. hell.. H55... the Tri-X cooler is massive, you may in fact see worse performance.
Save the money, get the Tri-X 290X, and make sure your case has decent airflow. That is my opinion anyways.