I demoed these a week or so ago in 170cm (I'm 5'8",150lbs, upper intermediate) - I would have preferred to try a 163, but they didn't have any. I really liked the ski - carved hard pack snow and ice like a knife through butter and was extremely easy to use - put it on edge, and let 'er rip. I'm pretty sure 163 would have been perfect for me. I didn't check the waist when putting them on, but was surprised that it was 74mm... it felt like a much narrower waist when carving.
For me, I like to do short carving turns - so I prefer a ski with a smaller turn radius.... I found it a bit difficult to do tight sharp carving turns..I'm sure a 163cm would have been better. I still found it a great ski though.
That's my experience as an intermediate working to become advanced..ymmv. One thing I learned when demoing, is you can't assume that because you like one length in one ski, you'll like the same length in a different ski. If you can't demo, I would look at the attributes of the AC20 (dimensions, stiffness, etc) and compare them to the SC74... that may help you decide if you want to go a bit longer or shorter with the AC20.