You'd have to dig around in the Futaba support forum http://www.rcuniverse.com/forum/forumid_125/tt.htm to find it but there was a reply a year or two ago by Krysta (Futaba rep) that describes why the S148 and S3001 are considered "precision" servos. There was also mention that they handle vibration slightly better than the S3003 and S3004 so they are recommended for helis and the S3004/S3004 are not.

Either servo is plenty for your trainer. S148 currently sells for $13.99 and S3003 is $10.99. I'd put the cheaper servos in the trainer.