If you want some ski that is fun, but also does well at speed and hardpack, there's no questions asked for me to go with the G31. The thing is tho, the G3 for this season is the replacement for the G31 and isn't as good as the G31. A bit softer, and I think the sidecut is different too
If you can find a pair of G31's at a shop, definitely get them. They're last season stock so they should be ALOT cheaper than the G3.
The K2ModX Pro you can still get this season...it's sold as the K2 Axis X Pro. 99% identical the the ModX, but doesn't have the piezo dampening. Good ski, but imho, the G31 is still much better than the ModX.
I don't know what conditions you've skied the skis in, but G31 will do well on almost anything, even hardpack. It's stiff so you're gonna have to work the ski alot to turn at slower speeds. The faster it goes, the better it'll respond.
ModX Pro is almost as good as the G31, but seems to be a bit more chattery on the hardpack than the G31.
X-Scream (series? 9?) If it's the series, it's a good ski that hasn't changed for the last few yrs that much...different prolink, but other than that, unchanged pretty much. Prolink system works very well on hardpack...and i'd put it up there with the ModX Pro. The thing is, though, i've seen about 8 pairs of Xscream series with a fractured core this past season...maybe it's just because they've sold more x-screams than mod x's or G31? iono...
Haven't tried the powertrac....
So if you liked the G31 and can find them...they're the best bang for the buck right now...if you can only find the G3...i dunno...try demoing it first and see how you like it. Also, which ski did you prefer out of the 3 demoed?