I've demoed both of those skis in the last week... and while they're both great skis, they're pretty different.
First of all, I consider myself an intermediate moving towards advanced skier... and 150lbs... my demoing was on small hills with a mix of light powder, loose granular, hard pack, ice... and I did one green, blue and black run with each demo.
The Xenon 8.0 really excels on soft/loose snow - it's a lot of fun, easy to ski, very forgiving...it's most at home doing short turns on soft snow. I also demoed the Xenon 9.0 the week before in similar conditions, and honestly, they felt pretty similar to me... the 8.0 might have had a bit better edge hold on hardpack, but it was not spectacular on hardpack/icy conditions.... They are both just as managable at a slow speed on greens as they are on steeper slopes... I decided I didn't want them because I ski mostly in Ontario and Quebec, where we have a lot of hard pack/ice...so I wanted a ski that excelled at that. I tried both skis in 163cm.
The AC20 I tried in 170cm, which was probably a bit long... still, I found them a lot of fun in all conditions - they carved hardpack, icy conditions like a knife through butter with little effort...and were also fun in a bit of powder. They have a very smooth feeling - with none of the skidding/chattering that I got with the Xenon 9.0 earlier in the day. If you ski mainly hardpack/icy conditions, this would be my choice over the Xenons. This would be a good ski to learn modern carving techniques. I personally like a ski with a shorter turn radius (like the Dynastar Contact Groove), but that's personal preference... I would have liked to try this ski in 163cm, but it wasn't available... I have to think I would have liked those even more.
As for ski length...one thing that's really hit home with demoing so many skis lately, is that just because you prefer a particular length in one ski, doesn't mean you'll like the same length in another ski. I'll let one of the members with more ski knowledge than me help you with that...