I own the 2009 i.Supershape Speed in 170cm (15.5 radius vs the new ones that are 14.3) and 2012 Atomic D2 SL FIS in 165 so maybe I can lend some insight. I am early 50's, 5'11", 165-175lbs, good but not great skier (level 8-9?), 70ish days/yr. I don't race but may start masters this year.
Very different characters but some similarities.
The Head is easier, a bit more laid back and GSy. Really fun to ski all over the mountain. It likes good technique but doesn't really demand it like the Atomic. Neither ski is particularly excited about being pushed sideways or skied in the back seat. The Atomic is a burlier feeling ski and SUPER springy. If you bend it you better be ready for the rebound. I like that the binding on the Atomic allows on the fly stance adjustment, it makes the ski more versatile. As you can imagine from the side cut differences, the Atomic is more turny. A better comparison would be the Head i.SL RD (which I have demoed) vs the Atomic SL or the Atomic Redster DoubleDeck 3.0 (which I owned for a while last year) vs. i.Supershape Speed.
On real ice, I'll take the Atomics every time. I'm more confident at 40+mph on the Heads, the Atomics start feeling a little twitchy or ready to bite. I think both are fun (but a workout) in powder but that is just me. The Atomics are a work-out in pretty much all conditions, great for getting exercise. From what I understand, you can get a more GS experience with a long length in the Head's and a more SL experience with shorter lengths (156cm to 184cm) but that isn't an option with the Atomic.
The tip protectors on the Atomics get a lot of questions on the lift which may be good or bad depending on your sociability.
My Heads are tuned .7/3 and the Atomics are .5/3 and I keep them sharp.
How is your technique? What are you trying to accomplish with this ski in your quiver? What is your height/weight/level/mountain?