Hard choice because both are important, but I would have to say gunner because the gunner can protect himself if he does or doesn't have a driver. But then the driver is important only if hes skilled at driving because he can escape a situation and save three or two people that are riding.
I was playing a game of CTF on Standoff and I went to the warthog and started driving. I drove around the outside to try and flank the other team and I did something really stupid. I drove by the invis spawn next to the tree and then over the base. All the while people where shooting me and my gunner with brute shots and chucking grenades. It actually worked too. When I came off the base I did a barrel roll and we went and got the flag. It's really important to have a good driver because if I had been a sucky driver in that situation, me and my gunner would've died instantly. While gunning is important, if you have a driver that sucks... you aren't going to get very far. While I prefer to shoot so I can get kills, I think I'm a better driver than gunner.