Brownsville will have somewhat cooler weather at times than Key West. There seems to be a straight path from the Canadian border to Texas, and some can even get that far south. Almost never down to freezing. Also potentially wetter.
Personally I'd prefer Harlingen or San Benito to Brownsville, but we have also stayed in La Feria, Weslaco and Alamo. All great places to stay.
I prefer a gated community. Not really worried about crime anymore than say Miami or Dallas.
Personally, I find South Padre better than Mustang Island.
If you like fishing, get close to the coast. Also SpaceX at Boca Chica is very active now with testing their next generation Starship.
Some amazing wildlife refuge, birding state parks and other natural areas in the stretch along the border.
We do our dental work in Nuevo Progresso, never worried about crossing the border there.
Full-Time 2014 - ????
“Not all who wander are lost.”
"You were supposed to turn back at the last street."
2012 Ram 2500 Mega Cab
2014 Flagstaff 832IKBS TT