Probably someone with more knowledge than I will want to know where you live and how much sun and use your pool gets. But since you are boarderline, I also think most will recommend the 40k unit.
For me, I went with a SWG that was rated for twice the sized pool. The main reason is that I am one of those people who run their pool pump the minimum required time and so need a higher chlorine output for the short time it is on. My PB was all ready to put in the smaller unit assuming that I'd run my pump 8 to 12 hours per day like most people. I went up in pipe size, up in SWG rating, and down in pump HP.