ORIGINAL: William Robison
For planes like yours I like the Hitec HS-225 servos. They are minis, with the torque of the standard size.
I've heard people *****ing about the 225's lack of centering (do a search on RCU for the details) but I have some and they seem to work just fine.
However, I'd go for HS81s on a plane of that size. They're plenty strong enough and, if you're really worried, just get some HS81MGs
Going back 20 years ago, I used a pair of Futaba S20 servos (about 24oz-In of torque) in an RC combat wing powered by an OS45FSR. This thing went like a rocket and those servos did the job absolutely perfectly. By comparison, an HS81MG is a mule of a servo with 50% more torque so I think it will do just fine in a 15-powered ship.