How to tell the lumen output of these LED's
Luminous Flux is the measurement you're looking for, and is measured in lumens. Looking at the pages you've linked:



The XP-E shows a luminous flux of only 425, while the XT-E has a luminous flux of 550.  Luminous Flux is measured using an integrating sphere, so it is the actual total light output of the device under test.  A given light, of course, may appear brighter than another with higher flux depending on optics and observer location, but as far as measuring the total light output, luminous flux is the measurement to use.

So to answer your question specifically, the XT-E is capable of more light output than the XP-E.  This also matches well with current consumption, where the XT-E consumes 1.5A at its rated output, while the XP-E consumes 1A at its rated output.

I expect that both would have similar brightness at the same current, so if you were to feed them 350mA(ie, about 116mA per discrete LED in the module) then you can probably expect to see very similar light output between the two modules.  If you're going to run them under current though, I'd suggest simply buying lower light output LEDs - they'll be much less expensive.

If you can explain what you're trying to optimize - be it brightness, efficiency, cost, etc then we might be able to provide better guidance.