A water test can be preformed by the county water & sewer utility company. My unicorporated community's district will do it free. If you live in an agricultural area, many USDA - Agricultural Extension Districts will also test water samples. I know this is true from my appointed position as a former local water resources board member in Maryland's (Atlantic Ocean) Coastal Bays.
I chose Vita of the three companies you listed. First because the dealer is also the service company. They are stocking parts and have been trained by and certified by Vita. Secondly because the wet test proved comforable to me (6'4") and my wife (5'4"). Thirdly because the website provides more indepth information, i.e., parts manual, owners manual, electrical hook ups, options for models, up grades. And the price was in my ball park too.
I live in a rural area and only two dealers are within 10 miles, several more are 25 miles away, 12 more are >30 miles from home. A total of 18 different companies, 28 models were researched. The pictures and websites are great, but nothing beat a good service company. Also, if the spa is of good construction, you shouldn't need any service for (we hope) five years or more. That is apart from chemical treatments and keeping the system clean.
My dealer/serviceman also helped with local ordinances (county and local building permits and electrical wiring suggestions). He also recommended (as well as the Vita website) sitting information and supporting foundation.
These things helped me, I hope it helps you!