You mean aside from the CPF mantra of buy both? I guess it depends on your wants and needs.
U2 - broader beam due to LuxV. Simple to control the six brightness levels with the ring. Output can be adjusted while the light is off. Still throw quite well, probably will have a donut hole due to the LuxV. The very lowest setting is not dim enough for some users.
Well made, should be rugged. As far as I know, only comes in black. Earlier ones were able to take rechargeables, newer ones seem to have a plastic sleeve in the battery compartment that doesn't allow this. Sleeve seems removeable. Has the SureFire warranty.
Gladius - more narrow beam due to LuxIII, it's a spot with a wide sidespill that is usable but dim. Switch is more complex than U2, but allows more options, such as the strobe. Output can be adjusted while the light is on, but what output the light turns on at is user choice. Has many levels of brightness that it scrolls through. Able to go quite dim.
Well made, though the CeraKote on the earlier versions has been replaced on the black model by HAIII, due to it wearing faster than expected. Other colors were possibly going to be HAIII in the future. Available in tan, grey, black and olive drab. It's not hard to put in batteries, but it's not your standard just the screw the tailcap assembly on and go procedure. Ergonomics are awesome on this light for a variety of grips, though some don't like the flanges on the tailcap. I love 'em though. Not the best light for rechargeable cells, it was designed for primaries and has a low voltage indicator that flashes the light every fifteen seconds with low voltage and that seems to be a factor when using rechargeables, even fresh off the charger.
I don't think you'd go far wrong with either light. I EDC the U2 as the beam is more suited to the close up work I use it for most times, and the output is adjustable while the light is off. I actually like the Gladius more, but practicality for my use the U2 wins.
Just my two coppers worth(less).