I think you should stick with the smae instructor unless:
A) You can't learn something the way he teaches it
B) You just HAVE to fly and he can't take you up
The reason I say stick with one is, because the other one might do stuff different ways and teaching you different ways. I think you can learn different techniques after solo. That way, you learn the ways you can do it, and then if you can't use the new techniques, then you can stick to the ones you know.
Hope this helps
Chad