Hi Andrew,
We install and service lots of spas and all have their good and bad points. My advice would be to look at the aftersales service. Do they keep the parts in stock, how long until they can come out and fix any problems. I find it really easy to gets parts from arctic and long winded from hydropool.
I still don't understand the salt systems fully and find it difficult to explain to customers. The more options the more that can go wrong. Look for things that will break once the warrenty has run out. Covers, cabinet panels, head rests etc, and how easy they are to replace or restore.
Good luck with your choice.