Not the expert here but a couple quick thoughts:

1) If you are only traveling to San Francisco and Florida, you probably have a pretty good idea of where you want to stay and how the properties stack up. If that's the case, I'd just stay where you want to stay and take the concomitant loyalty perks at whatever chain that happens to be

2) 4PM checkout at starwood isn't just for platinums. Gold and (I think) preferred get it too. But it's not guaranteed at resorts or conference centers. Since it sounds like you have a very small list of properties you'll stay at regularly, I'd suggest you confirm they are in neither of the above groups before you make a hotel chain change for this benefit

3) Upgrades. I can't speak from much experience for Florida, but I have experience in SF and domestic U.S. in general and even as a plat you will probably be disappointed in your upgrade rates. I am around 5-10% weekly but these are Monday-Thursday peak time stays. I usually get upgrades on the rare weekends I burn points to stay somewhere for personal stays.

4) The 24-hour check-in/check-out cycle is called Your24 and is really great. I use it every time I fly a redeye to the east coast, and occasionally otherwise. It's a difference making benefit to me. It also requires 75 nights in a year, which is a bit more than garden variety starwood platinum, so plan accordingly.