I own both a T3 and a XO3, and to me both beams look absolutely identical. Ditto output and throw. The only difference is a slightly different tint.
I slightly prefer the T3 because of it's better grip. The clicky is nice, but does not make that much of a difference to me as it does to other people.
The tailcaps are interchangable. There's another thread which mentions that Inova sells the clicky for the XO3 for 20 bucks.