I own both. I carry the U2 for work almost all the time now. Both are good lights, IMHO.
Beam: Gladius beam is tighter, but has a wider, albeit far dimmer sidespill. Their is some "star pattern" in the hotspot and a few minor artifacts, being only slightly darker. I rate it a good quality beam. It really throws well.
U2 Has an overall brighter sidespill, it's a wall of light. There is the donut to consider, and mine is slightly noticeable in real word use, but not bad.
Both the Gladius and U2 are extremely white tints, with a slight edge going to the Gladius on high. On the lower levels the U2 stays whiter, though both go slight green. YMMV.
Controls: The Gladius allows for a dimmer level, and more of them. The Gladius options are nice, modes and dynamic changes are pretty good.
The control ring of the U2 is simpler to operate, and more precise. You can change levels with the light off. Although the Gladius has more levels (100?) it's ratching up and down with one button controls is less precise, i.e. it's easy to go up or down too far compared to the U2 and it's ring. And as far as I know, you have to have the Gladius on to change levels.
Ergonomics: Gladius is better balanced and feels better in hand, over or under. U2 is not bad mind you, but seems front heavy. The Gladius feels like an extention of the hand.
Durability: I've some concern over the Gladius tailcap, especially now that I've heard of it breaking in lock out mode when dropped, and that's the way I carry. U2 seems rock solid, and although I would not want to drop either, I'd rather drop it than the Gladius. Warranty edge would also go to SureFire, but I think both are good ones. Gladius finish on my OD has not been an issue, but I baby it.
Price: Gladius might be had for a bit less, depending on where you get it. I rank them close though.
BTW, having been strobed by the Gladius from 15 feet away, in house with curtains drawn in the daytime, I can attest to it's disorienting effect. Intense!
In short, the warranty, simplicity and wider beam make the U2 a better work light for me. I'm not sorry about purchasing either, but the U2 refuses to stay home.