I have a Circupool Si-40 (which I think is the same as a Si-45), and I am running mine effectively for 3.2 hours a day (actually 45% with 7 hour pump time split into two periods) in a pool about half your size. I'm guessing that in your Arizona pool it would have to run at 100% for maybe 10 hours in the summer. If the comparison table at discountsaltpool is correct, the Circupool Si-45 can generate 2.0 lb of chlorine a day, whereas the AquaRite 40k can generate 1.4 lb. So, I would expect the AquaRite to run at 100% for 14 hours. If you get the Pentair IC60, that can generate up to 2.0 lb of chlorine per day, so it would run for perhaps 10 hours.
So, figure up the cost of 4 hours per day of pump run time over the life of the cell (although spring, fall, and winter may not have this difference) plus the cost of the shorter life of the smaller cell. Let's assume the average run time is 75% of the summer run time. That would mean 3 hrs of pump run time x 365 = 1095 hours per year. That will cost you about $150 per year with a 1.5 hp pump. I think all the cells are rated for 10,000 hours of operation (580 lb chlorine for the T15 and IC40 cells, 830 lb for the IC60 cell). The IC60 cell will run about 2700 hours per year, so its expected life is 3.7 years. The AquaRite 40k will run about 3800 hours per year, so its expected life is 2.6 years. The Hayward T15 cell costs around $450, and the Pentair IC60 costs around $765. So the yearly cost of the cell is $173 for the T15 and $206 for the IC60. So here is the summary of the annual cost comparison:
Hayward T15: $150 + $173 = $323
Pentair IC60: $206 per year
So, if my assumptions are close to being correct, the Hayward will cost you about $117 more per year to own than the Pentair IC60 (not including the original cost of the equipment). I would go for the bigger cell.