Im thinking of purchasing a bike light. Which of these LED is better T6 or P7?

XMLs are significantly better than the P7s

XML hands down.

Semper Fi

Yeah - and I want to know when the "next big thing" is coming in LEDs

Is there any kind of "lot code" or other changes yet in the Cree "XM-?" line up?

I know there is more than one version...... is there a good CP thread? or some article you guys know about?

there is always something new. They have come out with the XT-E and XB-D. I think the only way to really keep up is to read the Cree site occasionally. CPF also has a forum about emitters where they obsess over them. I never really saw much reason to worry about keeping up too closely. XM-L is a big step up over a P7 or an MC-E, but they all have their places.

Not all LEDs of one "model" are created equal. The "base model" you're discussing is the XM-L, but when CREE manufactures them some end up being relatively brighter and some dimmer. They separate these during post-production testing into "bins", T3, T4, T5, T6 for XM-L. There is also at present a U2 available which is about 7% brighter for the same current/voltage, but it has limited availability. A few high-end flashlights come with them, and you can get the emitter + board from Cutter Electronics, but I don't know of any bike lights with them.
Here's a reference for quickly comparing different LED's. They also have pages for other manufacturers.
https://flashlightwiki.com/Cree
The newly-released XT-E ad XB-D lights have lower current limits than the XM-L. While they are brighter at lower currents, if you're using a single LED you can get more light overall out of an XM-L. If you're using 2 or 3 LEDs, using these smaller, much cheaper LED's can be more cost- and power- effective, as well as enabling a more tightly focused beam. But at present, there's nothing in the immediate future to supersede the XM-L.

Regardless of which one is a 'better' LED, its the design and build quality of the driver assembly, heat sink housing, weatherproofing and power supply that determines if you end up with a light worth buying. I personally wouldn't buy a car based simply on how many cylinders the engine had and lights aren't much different.

I personally wouldn't buy an otherwise nice car with a [insert your own expletive here] engine.

CPF? Why not budgetlightforum.com? You'll find out when the affordable flashlights are available. Your best bet for off the shelf flashlight is going to be XM-L.

Is there such a thing? Currently the only decent cars I'm personally aware of that might have engine issues would be USED cars.

Lights, on the other hand, can have a wide range if quality depending on who builds them regardless of what LED was used.

https://www.pcmus.com/Dive-Photos/High-Power-LED.htm

On this page, for example, I'm not seeing any earth shattering advantages of one type over another. Except possible the ability to build in series An 'advantage' that can be completely negated by the design and build quality of the lens and driver assembly, heat sink housing, weatherproofing and power supply as already stated.

I'm running P7's myself and it was a deliberate choice in spite of whatever else is available. Price wasn't any part of that decision.

My own experience has been that 'bargin basement' lights are more likely to have issues - regardless of LED type used.