Hi, a 20 X 45' would allow you to have the deeper pool and still have a 'standard' shallow end ~ 10 -12' without a hugely slanted transition slope. The deep end 'hopper' walls would probably change from the 4' standard to 6'.(again for ease of troweling the hopper walls)

With shot concrete pools, you have a little more leeway - you don't have to make the walls fit ~ perfectly to a premade liner and the material can be shaped into whatever you want