We've had two so I'll give you my take.
Determine how big you want it and do you want one that requires a separate 220 line or one that can run off a standard 30amp 110 connection. As I said we are on our second one, both have been the type that run off a standard 110 connection. First one I honestly don't remember the brand we had for 11 years, it was a 5 person with lights and pretty basic otherwise. Biggest issue was that it used the pumps to actually heat the water instead of having a heating element. This caused a lot more wear on the pumps and was ultimately it's down fall a few years ago. The pumps died and it was going to be $1000 in parts plus labor to replace them. We got it just before the kids and used it a ton when they were little, we'd put them to bed and go out for a soak and a glass of wine. As they got older and had busier schedules we didn't use it as much so when it died we went without one. Enter the pandemic and my wife mentions she sort of misses it, my younger son is also now playing more competitive travel and high school soccer and has some soreness so we decided to use our stimulus checks and get a new one. At first I was going to bite the bullet and get the connection rewired to 220 because I didn't want to deal with the pump issue again. But after talking to the local pool place I found out the newer ones that run on a standard 110 connection have separate heating elements. So we got a Freeflow Excursion (
https://www.freeflowspas.com/), it is a 5 person with lights and waterfall (they pretty much all have them) but other then that a basic hot tub. We got it in April and have been thrilled with it, can't wait for the first snowfall because that is one of my favorite things to sit there while it is snowing around you. My son will use it when he feels sore after a tough weekend of games. The new one also heats up much quicker. I generally leave it at 100 degrees (during the cold months, cooler in the summer) and if I know we are going to use it will turn it up a couple degrees a few hours before. I normally like about 102, I find 104 is too hot unless it is really cold out and we know we are going to sit out there awhile with friends or something because they do cool off a little when you have the jets on full for awhile. I believe they all cap you at 104 degrees now.
On the sauna topic I don't see how yo people find that enjoyable, have tried them at different resorts we have been at and sitting there just sweating for the sake of sweating is not for me.