The main reason I fell into using UGS on the XXL is that it works really well with the Triquetra touch plate, which I use sometimes but not always. UGS also jogs very nicely using a wireless keypad (including quickly changing the jog step size). It isn't unusual for me to type G90/G91 commands right into GRBL (most common is g90 g0x0y0) though. I'm one of those weirdos who actually likes the command line. I've actually done simple router-table like cuts (for example routing t-tracks into MDF) just by sending manual commands to GRBL. The point is, your mileage may vary.
That said, with both a Nomad and the XXL -- and being the kind of hobbyist-guy where it isn't uncommon for weeks to go by without working on anything related to CNC at all -- the XXL is mainly for "rough and ready" big stuff, and Nomad is there for anything precise. Precise and big rarely go together, especially if the design itself makes sense. Nomad is precise and accurate without any fidgeting, Shapeoko not so much. XXL is a big sucker though and makes short work of large objects. I still have my first CNC - an original Othermill - which I use strictly for PCB's, which it does straightforwardly and accurately (again without any futzing).
They are very complementary for me, though I'm sure folks with the time to put into futzing with it can get Nomad-like results, and great PCB's for that matter, from Shapeoko. The common denominator of my setup is that I'm lazy, so when I have a project to do I like to get right to it with minimal tweaking.