Both skis you're considering are excellent and float very well. I have experience with both skis, as well as several other good off trail skis.
The 2006 - 2010 Yamaha Mountain skis you spoke of are very nice and are pretty much a copy of the SLP Powder Pro Skis. They are 7 1/4" wide and with proper shimming 1/4", being set up toed out 1/4", and with adequate wear bars they float well and behave adequately on the trail. I've used them alot. IMHO the best ski ever put stock on a Yamaha sled. Why Yamaha went away from them to the MT-9 or what ever it's called in 2011 I'll lever know.
The Simmons Gen 1's are IMHO the best all around trail ski I've ever tried. All darting is cured. I toed them neutral, shimmed them 1/4" (to reduce ski pressure), and put a 1/2" wear bar on all 4 places with 8" carbides. They behave very nicely and I may be going back to this set up on my Attak GT shortly. We'll see. The only draw back is the great funneling effect which makes them float so well can kill a guy on the trail. When there is bad slop on the traail the skis can funnel too much and they can lose their bite if there is ice under the slop in the corners. Trust me, been there a few times and that is why I'm now using C&A XTX's now on my Attak. Then Gen III's will float even better for off trail.
C&A XTX: These skis rail on the trail period. No darting and no pushing at all. These rail like a snowcross ski, but are so wide they do not sink -- in fact, they are 7 1/4" wide, making them float very well. Very tall keel too. May be the ultimate ski except they can steer hard and they are on the heavy side.
Slydog Powderhounds: What I'll end up putting on my Apex MTX, replacing the 2010 Yammi mountain skis. The Slydogs float the best of anything I've tried personally. They are also very predictable and forgiving. But they can push in the corners, which is an issue many guys have with these skis. Just be aware of that when trail riding. But they are a great cruising ski and float like a dream.
My personal opinion based on the above is try both skis and see what you like. I like the Simmons better than the Yammi Mt skis -- but for just Mountain riding you might be happy with either. But in the Mountains I'd use the Gen 3 Simmons over the Gen 1's personally. But for $100 you really can't go wrong with the Yammi Mt skis. If money isn't tight get the Slydog Powder Hounds, which will beat either off trail.
