i own the GR and i love it, it can sound very clear but it can also colour the sound drastically. it's built like a tank.
the UA pre looks nice but i can't stand the wobbly plastic knobs/pots, dunno about the sound but i would honestly not jump on the "tube" train.
works perfect for me, i love it on dynamic mics, where i only turn up the gain but leave the output down, this gives a nice kind of "saturated tone".
the HI-Z input is great also, it works great as a DI
It's not really a neve "clone", it's inspired by but not really a reproduction like most others out there. It's definitely the one i would go for between those two. Impedance switch is a rather nice feature to have.
If you drop the output knob all the way back and drive the gain harder the GR gets some neve-y thickness. If you dime the output and pull back the gain it gets quite clean. When you add in the additional variation of the impedance and loading it really shines as a flexible work horse preamp. There is nothing wrong with the UA, but the GR is a modern classic.