I spent a full day at Mammoth on each ski on back to back days with exactly the same conditions and exactly the same temperatures and here are my results.
The First day, I rode the CX 80. The day started out on the very hard pack side and turned into very soft conditions by mid day.
The Cx 80 is a great ski and I found my self really enjoying laying the ski down and running some quick edge to edge turns.
The ski seemed really versatile on the type of turns I could make and it has enough spring to really crank out some slalom turns while having enough grip to run some GS turns.
The ski, however, does have a speed limit and did not fare well at speeds in excessive of 60mph.
Another downside to the ski was how it handled a couple inches of slushy snow. I found it almost impossible to keep both skis doing the same thing.
The second day, I rode the AC50. The day started out very hard pack and like the previous day, ended on the slushy side.
The AC50 is not as playful of a ski as the CX 80. It did not initiate turns as fast or with as much spring. However, there is no comparison between how the AC50 and the CX80 handle speed.
I wear a GPS when I ski and on the first day, I had only one run where I broke 60mph and my max speed was 61.3. With the AC50, I had seven runs with speeds in access of 60 mph and some runs breaking the 70mph barrier.
The AC50 has a very stiff tail in comparison to the CX80 which allows it to handle sliding better than the 80 which becomes unnerving when you need to slide the ski.
I personally will be looking for a pair of the 50s this off season. However, if I lived on the east coast or somewhere with narrower shorter slopes, I would definitely choose the 80s.
PS, The 80s felt like a gown up pair of the Bandits:)