Alright mate, I do the Glentress trailquest each year – so hardly a TQ veteran, but I would definitely recommend giving them a go. It's a great way of doing something competitive on your bike, without having to be super fit. The strategy aspect of planning your route adds an extra dimension to just riding around as fast as you can.
At the GT event, it seems that you get a lot of guys entering who come from an orienteering background, and don't really know how to ride a MTB. So it's an interesting mix of competitors – there's a few seriously fit MTBers who clean up, but the body of the field aren't your typical MTB crew.
It'd be interesting to hear from more seasoned Trailquesters – I do the GT event because it's local and I know the area well. As a result I probably enjoy it more, I've never done one sight unseen where I had no clue about the trails. It would no doubt place more of a premium on reading the map and really taking stock of the contour lines.