ALL branches of military pay the same, however each branch has their own helicopters. If you want to be a lift mechanic(Blackhawks, Chinooks, and Sea Knights) all branches have those. If you want attack(Apaches, Cobras, and Kiowas) Army and Marines. However you do not get to choose right off the bat for the Marines, they choose the job for you, so you may get infantry.
Again, all branches pay the same you will not get more or less money for any job due to a different service, other wise everyone would be trying to get into the best paying service, it is the unwritten benefits that make one more desirable to a person. If you really want more money go into the Army as they deploy more and longer(good for single people), if you are married, I would recommend the Air Force as they do not deploy as much.
I am a Army OH-58D Kiowa mechanic and have 36 months deployed in 12 month stretches since Jan 2003.
sig pic courtesy of GBALT05