If, as your profile indicates, you are from Washington state and plan on hunting deer here, it doesn't really matter which caliber is better. The point is moot as only one of those calibers is legal to hunt deer with in this state. And that would be the 7.62x39. Washington state requires that a rifle be at least .240/6mm caliber to be legal to hunt big game with.
Personally, I don't think the 7.62x39 would be such a bad deer cartridge. Powerwise, it's about the equivalent of the Winchester 30-30. The 7.62x39 can also take advantage of pointed bullets whereas most 30-30 rifles cannot because of their tubular magazine. I've got a 7.62x39 AR15 that I am considering using for deer season this fall.
Do NOT even think about using EITHER of these cartridges for elk. They are both woefully underpowered for elk. .270 Winchester is the minimum I would use for elk, and even that is on the light side for such a large, tough animal. If you want a semi-auto for elk hunting, you'd better either settle for a Remington 7400 or save up for a Browning BAR.
I know it's not what you wanted to hear, but if you truly want one reasonably affordable rifle for both types of hunting, forget about the semi-auto and get a decent bolt action rifle in 7mm Remington Mag, 30-06, or .300 Winchester. Any of those three calibers will offer the versatility to handle anything you might run up against in Washington state.