You should buy coverage in the state where you officially reside. Most states consider you a resident if you intend to make that state your permanent home. So-called “snowbirds” may own a second home and live part of the year in another state, but their official state of residence is where they spend most of the year, where they pay taxes, where they register their cars, or are registered to vote.
If you are buying coverage in your state of residency but spend a significant amount of time in a different state, you may want to explore plans offered by insurers that use a national provider network so that you could find participating providers in more than one state. You could also explore insurers that arrange to cover as in-network other insurers’ network providers. (For example, some, though not all "multi-state" plans, and some, though not all "multi-state" plans have such agreements.) You could also evaluate what out-of-network coverage, if any, your plan offers.
If you need emergency care, but are outside you plan network, you will I still be able to be covered by your plan? Emergency visits will be covered the as in-network, regardless if the hospital is in your plan’s network or have a PPO, HMO, or EPO Plan.