I just posted this in your filter thread and realized I should post it here:
Like yours, the Aqua Rite has different cell size options that can be used with the same control unit. I believe the three options are 15k gal, 25k gal, and 40k gal.
IMO, while the 25k gal size would probably work fine for a 16k gal pool (though I don't know how the spa impacts this) the 40k gal size cell would offer the best value as it would last the longest.
Toy builder, though I only have first hand experience with the Aqua Rite, I'd have no problem recommending either brand based on what I've read.

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