RE: what is the difference
The PG 2000 is not heading hold, but the GY401 is (Futaba calls it AVCS). Also, unless it's the PG 2000 II (I think that's the number[sm=confused.gif]), it also does not have remote gain adjust.
If at all possible, get the GY401 with the 9254 servo ... it's super fast and super accurate!
Keith