I fly 40 and 60 size sport planes, and have used S148 Sutaba servos for many years. My latest radio came with S3004 servos, which also seem to work fine. The specifications for the two servos is identical except for a slight difference in weight. The S3004 is ball bearing, but the S148 is a little more expensive. Is there really any difference between these two servos?

I'm HOPING you mean futaba, not sutaba... if they are sutaba, must be some interesting chinese knockoff, sort of like kemwood car audio gear... (kenwood)

The s3004 has a single ball bearing and the S148 has dual bronze bushings.

Check out http://www.servocity.com/html/futaba_servos.html
They have all the info for checking out servos and their different features.