You can see the difference between the U2 and T6 bin here: See the #3 reply Difference between Cree XM L2 U2 and XM L2 T6

And a general discussion from a flashlight website: The XM-L is a line of light emitting diodes from Cree. There is a newer version, the XM-L2, which tends to be more efficient and capable of producing more light. As with other high-performance semiconductors like computer CPUs, there is considerable variation in performance from one unit to the next. After manufacture, LEDs are tested and sorted in to bins based on their performance. Higher bins produce more light from the same amount of electricity. Sometimes manufacturers of super-cheap flashlights get the names wrong or mention only the bin and not the product line.
Common bins for the XM-L and XM-L2, from less efficient to more efficient are T4, T5, T6, U2 and U3. A U4 exists, but is rare. At its rated maximum current of 3.0 Amps, an XM-L2 T4 produces about 930 lumens. An XM-L2 U3 will produce about 1240 lumens with the same current.

I am quite familiar with the inexpensive XML-T6 LED flashlights available for less than $10. The maximum real output for the XML-T6 bin is 1070 lumens according to CREE. It's my favorite bike light due to the low cost and being so rugged. Most allow you to focus the beam and have three to five settings for output and flash mode. I like using my Ultrafire brand flashlight with the 18650 battery because the batteries are inexpensive and you can easily carry a spare battery in a tool bag (properly encased in plastic so you never short a battery!) The flashlight with battery weighs 140 grams or about 5.5 ounces. The batteries are rated for 500 recharges. You can also get a similar light in red for a taillight but the red T6 bin only delivers 800 lumens. That's still way better than most bike taillights and cost a lot less. I also own a five CREE LED T6 bike light that runs on a separate Li-ion battery pack but that is much more trouble due to having to deal with the hefty battery pack. It's very bright though.