The main benefit of AC bearings is that if you get a matched pair, you just crank them up together and the preload is automatically set for you. I don't know how many people are actually using these matched pairs, I know I'm not so I carefully set my preload with the nut on the end of my screw. In this case, my opinion is that you really don't have any advantage over plain axial thrust bearings, and indeed I have also used these to equally good effect on ballscrew conversions. So if you don't get a matched pair, I'd go for axial thrust bearings if the cost difference is that big.
LongRat
www.fulloption.co.uk