Concrete or steel or aluminum ?

Thanks to all for your help and guidance.
I just got my 3rd quote and this guy is telling me I should do a concrete pool. 2 previous contractors said to do steel or aluminum walls.
We are designing a 12x24 pool. Shallow. 5 feet depth max. We are in the north east. NYC area.
This guy is telling me that concrete is far superior because the metal construction walls will warp or be wavy once the pool is backfilled.
Is concrete really necessary?