I've not used the Shure GLX, although I'm very familiar with Shure PSM and Shure ULX gear, and I vastly prefer Shure to other IEM and mic options. With personal guitar wireless, I've had experience with the Line 6 G50, G90, and the newer G70. I've also used a non-digital Shure PG which was total crap on the tone.
The G50 and G90 are essentially the same in different form factors (pedal vs rack). The pack is metal and heavy duty, and I've never experienced dropouts in an environment where there is a lot of WiFi, as long as the WiFi is controlled and managed. I also like that the battery meter is on the pack. I have occasionally used the newer G70. I really dig the multi-pack and output options. The tuner is nifty if you need it (I don't). The battery meter is very clear, but it's on the receiver, so you have to walk out on stage to check the battery levels (kinda dumb). The pack seems fairly heavy duty, but the clip totally sucks, so I just stick the whole thing in a pocket or else I gaff it to my strap. All the Line 6 options seem to be transparent on tone. I don't use their cable tone features because I usually hook up a 10' cable between wireless and the next device, so that if something goes wrong, I can switch to the cable quickly.
If there were a perfect unit, it would be the heavy duty pack and rock solid signal of the Shure GLX, with the output/pack switching options of the Line 6 G70.