If you don't subscribe to the 4k options then there isn't much point going for a 4k capable device so get the cheapest one (or get 2, one for each tv). Just looked the difference is currently £2 - get the 4k version.
I'd normally suggest getting an Nvidia Shield TV but they are £100 more than the fire stick, £149 for the non pro version but worth it in my opinion, I've had one for years and it gets updated every 8 months or so and keeps working. The firestick my son had gave up the ghost years ago but in your situation where you are moving it about the firestick is probably the way to go (and we did find taking the firestick on holiday was good, which would be a pain with the shield).