I went to Jet Tech as well. Can't say enough good things about their instruction, syllabus, instructors.
The only negative to me was the lack of tie-ins to SWA. When I went through, Jet-Tech had no interaction with SWA in official or unofficial capacity. The owner of Higher Power, I have heard, is on friendly terms with most of the people at the Dallas training center, has been used as a reference with SWA, and has close ties with some SWA pilots. Additionally, when I go to start at Southwest, it will have been over a year since I will have even seen the inside of a 737 sim. Both Jet-Tech and HP will allow you to audit ground school stuff again, but I ask you, which one is more conveniently located?
You won't go wrong with Jet-Tech in the quality of their training for the 737. It just seems like there may be a little edge with SWA in Higher Power.