I love the G30 (I ski the 193), but it and the G31 are much different from the current G3, as far as I understand. First, the sidecut on the G30/31 had a much "straighter" tail (92-mm) that didn't hook up as much as a ski with a wider tail like a Scream (96-mm). Some liked the narrower tail, because it allows the ski to run a bit straighter at high speed. I also think that it makes it a bit more predictable in the heavier crud (personal opinion). The G3 has a different sidecut (108-70-96) and a bit wider in the tail. I would imagine that this would make it behave more like the X-Scream, hooking up a bit more at the end of the turn.
Also the the G30/31 is the Volkl traditional Ti laminate construction, while the G3 uses the EnergY construction where basically a Ti cap is molded over the core. It still skis great, but it is by no means the same ski as a G30/31. I'm not sure, but isn't the core of the G3 a wood/foam combination, now?
Anyhow, with that being said, the X-Scream is the original do-it-all midfat. Everyone else is still trying to copy it. Is is also probably one of the safest buys for a strong Intermediate or more proficient skier. Get a Scream with a Sollie binding and get the extra year on the warranty. Who knows when those prolink cracks may start! ;^) (Sorry...I couldn't resist!)