Go with the T-15 cell. It will run on a lower output percentage. For Goldline units that means for every 100 minutes of pump time the cell will run for a certain percentage of that time and be off the rest of the time. The cell will last longer, you won't have as fast a pH rise, and there should be slower scale formation in the cell. I am actually running a T-15 cell on a pool that is under 7K gallons with no problems whatsoever! (I have an older Aqualogic that only accepts T-15 cells. Only the newer units can accept T-9 and T-5 cells so make sure yours can if you decide to go with a smaller cell.)