I think it’s personal choice. When I was deciding on a back up comp gun, I took a look at a used x-5 with the thought of adding the TXG grip, versus a second x5 Legion.
With regard to the original x5, what you don’t get versus an x5 Legion is the new trigger group/ FCU. Comparing apples to apples, if you hand Bruce an x5 and an x5 Legion, by virtue of design, Bruce (or in my case, Robert at SA) can produce a better feeling trigger for the Legion.
My decision was that I wanted the best comp trigger I could get my hands on for this platform. So I am willing to tolerate the gas residue at least for now.
If you’re satisfied with the x5 trigger work you have now, by all means grab the module and rock.
If otoh, you want an even better trigger, grab an x5 Legion and retain your current x5 as a back up.
Its really all personal choice.