I'll focus on the last part
If we are going to Snowbasin is it best to look for rental there or in SLC?
Would I be expecting Burton Fish as only option or do they usually have more selection?
You'll have a wider selection of boards looking around SLC, and probably a better rental/demo price. Milosport and Salty Peaks are both right next to each other, and pretty decent shops, so you might want to check them out or give them a call about demo/rentals. If nothing else you can compare their selection and cost to what you'd be able to get at Snowbasin.
Some of the other SLC riders might have some other suggestions. I've only been in SLC for about a year, but I bought some Raiden Blackhawks at a huge discount at Milo and they were a super cool store.