If you google it, there's an excellent review somewhere of The GX group set (perhaps blister gear review - I had a quick look on my phone but couldn't find it).
The review effectively pointed out that the main difference between the GX and more expensive options (X1, XO1, XX1) was that the body was cast as opposed to forged, which is cheaper manufacturing process but may come at the expense of strength. There is bugger all difference in weight.
I'm pretty sure the reviewer noted no difference in shift quality between XX1, XO1 and X1 derailleurs. However with GX there was a noticeable decrease in shift quality - less crisp and more difficult to set up. The suggestion was that there was no real performance or weight difference between GX and higher end group sets save for the derailleur.
Based on this I'd suggest going for an X1 derailleur. I'm looking at a drivetrain upgrade at the moment and will be getting a GX group set with an X1 derailleur.