My 2cents, buy servos specifically for your next plane. If you just buy some you may have way to much or not enough servo. I would hate to have to choose a plane because I only had certain servos.
When I buy a servo I think of strength,speed,weight and price. In that order,also I have not had good luck with Hitec. I buy futaba. Digitals are nice but if you buy them dont get the cheap ones, the only futaba/JR digitals I like are $70+. For instance I have some S3151's and they are basically a s148 that cant handle 6 volts.

A lot of folks use Hitec with good results, if your 15 and concerned with price you might consider them. Personally I wouldn't trust them in a High dollar gas plane.