Flexible for sure - much easier for the next water heater. Also I would have him install shut off valves on both legs so you don't have to drain down the system totally to replace the heater. This will be very important if you need to sweat new copper onto the pipes to reach the new water heater connections (nothing's ever the same on these). With valves on both legs, you shut them off, drain the last bit of pipe and go to work. Done in no time without fighting water in the lines. On mine I even put a crossover with a valve above the shutoffs, so all I need to do is shut off the 2 legs, and turn on the crossover valve, and I still have full water supply to the house, although no actual hot water. Makes life a little less annoying while it is down.
Tank water heaters only last 10-12 years, typically, so you will have to do this on an all-too-often basis. But a lot also depends on the water chemistry in your area. I assume you will have a whole house filter and softener since you will be on a well. The filters are critical to extending the life of the tank (removing sediment). They should be the first thing in the house, like right after the pressure tank.