I taxed my Prop1 to it's limits (which are considerable!) with my 2 axis skull set-up. A Prop2, which has a great deal more capacity would be a better choice of the two items. Really would be best if you jumped over to the EFX-Tek support forum and talked to Jon. There's a few folks over on the site trying their hands at 3 axis set-ups. I'm sure Jon can steer you in the right direction.
The Prop-2 has more memory, I have maxed out the memory on a Prop-1 on a couple of projects. Positioning servos takes memory as they have to be refreshed, I have not played with the Prop-SX yet, but I understand it is best for servos - there is also the Servo-Pal that can fit onto a Prop-1 or Prop-2 that does the refresh automatically. You will have to buy the Servo-Pal from Parallax (EFX-TEK is a spin off of Parallax).