The Vista is more comparable to the Caldera Geneva, not the Niagra. The Geneva and the Niagra are basically the same, but one has a lounger and one doesn't.
One of the first things you need to do is decide whether or not you want a lounger, the best way to do that is to sit in them dry, and then wet test if you have to.
All of the spas you mentioned are good, go see your local dealer, see who treats you well, and who has a good reputation in the area, sit in them dry, see how the jet placement is for you body and then narrow it down and wet test.