Wrestling will teach you how to get a guy on the ground and hold him there, that's pretty much it. Boxing will teach you how to get out of the way of strikes and throw back some yourself keeping you standing so in case another person shows up, you'd know. That is just scratching the surface though, if you want to avoid taking damage in a situation where you're forced to fight you want to be able to get out of the way and move fast, something that wrestling will fail you in as you can take punishment in a attempt to close the distance, not worth the risk.

The best defense is to keep your distance. Obviously, if you cannot, it is good to know grappling techniques and locks, but if your self defense fighting style relies on them, you are opening yourself up to be too close to your attacker. As Scotty said, both are useful, but boxing would probably be better overall because you learn to control distance, evade strikes, and the goal is to hit and not be hit, rather than to engage hand to hand, like in wrestling.

Boxing

Even though i box,I think wrestling is greater for self defense. Sometimes you wont hit someone hard enough to stop them, but if you can slam someone on the floor, they're pretty much done, and it looks like you won the fight.

Boxing. Basic self-defense techniques are holding your hands up, ducking, slipping, parrying and blocking a punch, all of which are taught and learned in boxing.

Both are useful.