I have the same size pool as you.

I went with the 60 cell and recommend you do as well. It will provide enough output for a pool your size and won't have to work as hard as a 48 cell, hopefully leading to a longer cell life.

I think it is worth a bit more on trade in as well.

You may want to check prices online.You might be surprised at the difference you can save from your builder's price.

Hope this helps