I've been riding Axis foils for about 3 years.
Started with a kite foil (K-series now) for a year.
Then tried some of the surf prototypes and bought the 820, which I kept for a year with a 720 as backup, which got used heavily for about 4 months.
Got to try a bunch of different wings, fuselage and stabs this southern summer, and I bought a 660/short fuse/370 stab which I've been on since February, on either 75cm mast or my custom 82cm mast.
I loved the 820; easy to ride, easy to learn things on, loads of glide. The 720 was the same with less glide but more fun carving around, which is what I enjoyed. (820 with the short fuse and smaller stab is better again though.)
And now the 660 is just better (for me). Lift and glide (almost) like the 820, but much more fun to hack and slash around on. IIRC it's a higher lift profile, so while it's like 1,100cm2 compared to the 1,500 of the 820, I feel I don't lose anything in the bottom end.
The 660 likes to be carving, while the traditional flat wings like to go in straight lines. 660 tracks around the carve, the flat wings feel like they "sink" or sideslip through the turn. I use a 10m Drifter when I have to, but usually use an 8m Switchblade.
So basically, I think it's the bee's knees
Haven't ridden an Armstrong so can't compare, sorry -- you might like to ask Gunnar. He's GMB13 on here...