I think you are better off going with one brand to start out with. You will get more help from a manufacturer if you are going only with them.
As a business decision, you will cut costs by only having one brand. Your parts inventory can be lower, you will buy the spas at a better price, you will build more coop dollars. One training program for your sales staff and techs, lower brochure costs (hot tub manufacturers typically sell their brochures and other marketing stuff). Other reasons are out there, but that is a good start.
Find a manufacturer that offers spas in a wide price range and you will have all your customer's price points covered.