I'm in a similar boat to you as well. Thinking about getting back into nitro for the occasional bash and these two seem to be the cheapest way in. Like you I haven't been able to find too much info on either. I'm no expert on the topic but this is what I've found so far.
There is a review of the XTM at rcuniverse.com. But keep in mind this is the american version which I think comes with a different engine. In Oz, the XTM comes with a Force branded motor which depending on who you talk to is a good value entry level motor, or a completely unreliable waste of money.
You can also download the manual from http://oos.modelengines.com.au/comma...key=meng&f_id=
I think the XTM has threaded shocks (so you can adjust ride height) and has a bit more ground clearance than the GS which I like. However the one thing that I don't like about it is that it has a planetary gear diff which looks a bit weak compared to the normal spider gear diff you find in pretty much everything else (personally a bit wary of diff problems after previously having a HPI MT2).
The engine in the GS from what I gather has a much stronger reputation. The quality of materials from my very brief looks also appears to be a bit better than the XTM. However the impression that I'm getting is that the Avenger was a bit of a flop for GS (and so has become their entry level model) and quickly brought out the CL-1 which is their main buggy. The GS also works out a bit cheaper as it comes with the glow igniter and fuel bottle as standard (you need to pay extra to get with the XTM).
Hobbyco is the GS importer in Oz. So its convenient (located in Sydney CBD), but they are also a shop that seems to concentrate on RTR kits so I'm not sure what their parts support is like.
I personally haven't made up my mind yet. But also other buggies in the price range you might want to look at are the ThunderTiger EB-4 (I think some shops are selling older versions for $450), and the Great Vigor Rex (about $450).
Keep us all updated on how you go, and I'll post any more info that I can find as well