The TK20 is not a bad replacement for the ProPoly Lux. I gave one to my daughter to replace her old ProPoly.
Although it is a much sturdier build, overall weight when fully loaded with 2xAA batteries is a bit less than the ProPoly with 4xAA batts. Output of the TK20 on Low is pretty close to the ProPoly, but of course the TK20 doesn't throw as far on Low (the Propoly has a larger and deeper reflector, producing more throw for a given output). But on Hi, the higher luminous flux Cree in the TK20 will out-throw it.
Runtime on Hi is reduced due to the 2 cells on the TK20, but since the Cree is more than twice as efficient, runtime of the TK20 on Low should exceed the ProPoly. And alkalines are still an acceptable choice - if you plan to run the TK20 on Low most of the time. And can always pack a few extra spare cells for more runtime.
Bottom line, it comes down to how you feel about the TK20 build (i.e. may be a bit more of a tank than you want - although others like that about it), and the warmer neutral-tint emitters (again, some like it, some don't). My
color and tint comparison has additional tint examples to help you compare.