GM dealers as a rule don't rebuild transmissions unless it's a warranty repair and GM makes them do it if it is cheaper to fix than replace.
A 4L60E from GM should cost about $2300 at the dealer plus labor. 2WD cheaper than 4WD on the labor side.
Both GM and Jasper have 3yr/100k parts and labor warranties on trans and engines. I would buy the GM if possible but would not be scared of a Jasper trans. Jasper also offers a HD build at an extra cost, as does Ford.
The belt comment is a very good one. I have seen "locked up" engines replaced over a seized alt/compressor/tensioner etc.