I have both the 1515/2.5d and the 1521/1Y. They have both been in my revo. running on 5s and 6s. I setteled on the 1521/1y for the revo just because it was WAY to much motor for any other vehicle I own. They both run without any heat issues on 5s and both get warm (150ish give or take depending on the gearing) on 6s, but still no issue what so ever. In the monster truck I like the feel of the big motor. It winds up a litlle slower than the smaller motor, but it is winds up way faster than the tires can keep grip or send it wheeling onto its lid.
My revo is fairly light for a MT, but I would have to say that either motor will be plenty for your Savage. Neither one even breaks a sweat. I also have a 1512/2.5D which gives a little snappier feel than the biggger motors. I actually like it better for a track motor on 4s than the big motors on 5s. It winds up quick but has a little more reasonable power band to control.
If money is no object then the 1521 is sure a hell of a beast. IMO it just looks bad ass
. Pop the lid off and people ususally do a double take at the size of it. Most folks are used to seeing a MM or Novak motor when you say brushless. The 1521 is easily twice the size of a MM motor.