it really depends on the actual board and your ability when it comes to surfing. I use the same boards for both although for me the 5'11 FW is a bit small for me to paddle at 6'2 and 85kg. the 6'3 Resin8 is my go to board for double overhead on the kite and overhead barrels surfing. this is a surfboard with no strap inserts!
for your size the 5'11 FW will surf fine as long as the waves are decent as the board has quite a bit of rocker, its the Taj pro model but with extra reinforcement and strap inserts. unless you are of pro level surf ability you will not notice any difference in feel to the standard Taj Firewire 5'11.
if your waves are a bit fatter then the new 5'8 dominator would be the go especially if you ride in lightish winds with the kite but if ride mostly with 7 or smaller then the 5'11 is sweet!
Best idea is to try get a test http://www.leepasty.co.uk