they are 2 systems that both work in their own regard, the ACE may be easier overall to maintain but both will work, ACE and ozone could be used in conjunction with each other if you have a tub that will be a very heavy use tub. A gun to my head I would take the ACE, the soaking experience is very nice, the water is soft and as long as you start-up the system properly its super easy to maintain through the life of your water.
Obviously both tubs are Watkins products so either way your getting a good manufacture backing the product you will eventually choose and since we only carry Hot Spring now I'll tell you what I like about the Flair (obviously some of these features are on both tubs): 24hr silent circulation pump + 2 jet pumps, 2lb FiberCor insulation, 5yr. component warranty, ACE compatible, comfortable lounge with calf and foot jets and now for 2012 they have the all vertical Hot Spring design cabinets which are very sharp. A lot of nice features for a very fair price if you compare it to other tubs in the same range, this tub has been a pretty hot ticket, I know I've sold 3 this month and working on my 4th
