Wattage, just get whatever you need for your build. As for which series. Anything on the lower end (CX/M, CS, VS series) isn't that good. Buy EVGA instead. They make better budget PSUs. RMi/RMx is the best bang for the buck. 80+ Gold efficiency and build quality is 10/10. Up to 1000W. HXi is more efficient and slightly more expensive. 80+ Platinum, quality is also 10/10. Up to 1200W. AXi has full digital power regulation. Other than that it's the same as HXi but a lot more expensive. Also 80+ Platinum. Up to 1500W.
Yeah my brother runs an AXi1200 and I run an HX1000, I was planning on an AXi1500 but seriously I would have 1100 Watts unused on my build the AXi1500 is just insanely OP as is...it would have added an extra £150 to my build over the PSU I went for which was the Corsair HX1000