While Union tank tees have a slightly higher up-front cost than non-union tank tees, union
tank tees will save you money in the long term. It is not a matter of if, but when, your pre-charged/bladder tank will fail, union tank tees will save you hundreds of dollars in the cost of labour. The few dollars you saved by purchasing a solid (non-union) tee will cost you, as you will need to disconnect both line connections and the wiring from the power source and to the pump to remove the tank tee assembly. With a union tee, all of the components can remain in tact on the union head, and the only the head can be disconnected from the tank tee leg. Then you can remove the leg for changeout of the new tank. We are seeing a continuous shift from non-union tees to union tees.
There are no specific applications that would require a union tank tee or a non-union tank tee.