Hi,
In a few short words - Warranty! Regardless of the spa that you choose there must be a reptutable warranty service station in the area. If you live in a rural area make sure that your closest service station has made a decent name for itself and will still be operating during the duration of your warranty.
Don't be afraid of extended warranty programs, I run a large service department and I have seen many calls come in for spas that are four and five years old that still have a full warranty, that really makes my clients happy.
We carry the Jacuzzi Premium brand and they have been very good to us. We sell and service these products to our clients because they work and nothing else. When it comes to ozonators, covers and other accesories all sps should come with a cover any spa that does not is simply cheap. I highly recommend ozone and a cover lifter if you have the budget for it.
I hope this helps
Jason