I did a considerable amount of reading as well as chatting with Justin about it. The main reasons I went for the Rupes over the Flex are:
Noise: the Rupes has been reported as considerably quieter than the Flex.
Stroke: Even the LHR15 is almost double the stroke of the flex, from my understanding this translates into faster correction.
Pad sizes: I can go right down to 5" pads which are easily found from multiple suppliers. I also ordered a 6" backing plate for 6" pads (like those purchased from Justin). The backing plate is also very easy to swap out. Actual width of the 5" backing plate is 4 7/8" and the 6" is actually 5 7/8"
Forced rotation: I am still not 100% sold on the idea. I like being able to vary the speed of the pad rotation as I have found that different pads work better with a bit more pressure and less rotational speed. I also figure in the long run that the Rupes has less moving parts and therefore more reliable
The Flex does have a slightly more powerful motor @ 590watts vs. 500 watts. But after my research, I feel the 90 extra watts are negated with the parasitic draw of the forced pad rotation system
Cost wasn't really a factor as I wanted to go with the 15 vs. the 20 from the get go. I paid $369.95, but also received an additional backing plate for free ($43 value). The LHR20 is more @ $399.95. When you factor in the 4 free pads or the backing plate, it's still a very, very similar value to the Flex (XC3401). I was extra lucky since Kevin through in some free Rupes pads too
Also weight was identical between the two, I was hoping one was lighter than the other...
OPM is an overall lower scale on the Rupes than the flex, to me this allows some safety for my skill level.