We've been alternating years recently...not going this year.
Scottsdale seems to have the most stuff going on, good dining options along with a pretty robust nightlife if you're into that kind of thing. Also lots of nice hotels in that area and some hiking options. Now that I have a kid we go to one of the hotels that has a nice pool which keeps her happy. The botanical garden and zoo are pretty good.
I agree that Salt River is the nicest of the ones that I've been to -- spacious but still intimate. Nothing to do around the complex unless Medieval Times is your idea of fun haha. The Brewers are so lucky to have Maryvale which is an awesome facility and centrally located...not an area with hotels but if you're not afraid to put on your gringo tourist hat and walk into some Spanish speaking places you can have a good time after the game around there. We found a paletas place last time which was perfect for a postgame snack.
Sloan was basically a giant zoo of Cubs fans, there were like 15,000 in attendance for the Brewers/Cubs game that I was at. I probably wouldn't go back there, it was too crowded and didn't feel like a spring training atmosphere.
The Mariners/Padres facility is my least favorite, it's outdated and way off on the west side. However, it does have a Portillos and a Culvers nearby, which is a big plus for those of us that do not have such luxuries near our current residence.
I know there are differing opinions about Phoenix...for me it ranks near the bottom of US cities that I've been to. It has sunshine and baseball in the spring and it's easy+cheap to fly to. Everything is too sprawled out and you end up spending half the trip driving around avoiding the other maniac drivers and wondering who thought it was a good idea to build such a place in the middle of the desert. But just writing this still makes me want to scratch my baseball itch and not wait until Opening Day.