The quality of the PCB and the heatsinking is very important.
In my experience RapidLED has done a fairly good job of using good components in his construction. His LED PCB(the board the LED is mounted on) are custom and quality. I would imagine the other 2 sites have similar quality.
I can't speak for the other places, but be wary of the chinese sites. They typically are using lower quality PCB boards which have lower quality heat sinks and traces.
You also have to realize that there are typically multiple "qualities" of Cree LEDs. Look at the datasheet for a Cree XM-L2. http://www.cree.com/~/media/Files/Cr.../XLampXML2.pdf
The U2 is significantly more efficient than the T5(~15% more light output). I know that RapidLED lists the binning code. You have to be careful with those aliexpress sales. Frequently they won't list it.