Have owned 120ED and ES 127 Carbon Fiber.
The star test on the ES showed a scope that had fair optics but nothing special. Maybe 1/5th wave of SA and a zone at 50% radius. A few people have asked for the test of their ES scopes and the two that I have seen showed optics similar to the one I had (nothing special). Mine was the Carbon version but it still weighed in at 17 lbs and was much nose heavier than the 120ED.
The 120 optics were better. Star test showed no zones, no astigmatism, and a very good SA correction 1/8th or better. This was the third ED refractor I have owned from the company that makes these and all have been quite excellent.
I found that the 11 Lb 120ED was comfortable on light Alt-az mounts that would not be close to being able to handle the ES 127 but if one already has a decent GEM mount, this should not be an issue. If a mount has to be part of the acquisition cost plane on spending more money for the mount that will hold the 127E.
Last but not least, and this is an important difference (unless changes have been made since I owned each of these). The ES has a very limited amount of focuser travel. It utilizes a two part focuser tube. This is done so that a 2" focuser (unless this has changed) could be used and not vignette when a focal reducer was used. The good thing about this was that with the extension tube removed, I could reach focus with the ES using binoviewer with a T2 diagonal and the BV (a Mark V) connected directly to the diagonal, and not have to use an OCS or Barlow. The 120ED had far greater focuser travel so pretty much any eyepiece could be used but this could limit the amount of reduction you could use before vignetting. It would not reach focus with BV unless an OCS was used (though someone I think made it work with BV by replacing the focuser, but this only matters if you thing you might want to use BV one day).
Overall, I liked the 120ED better simply because it could get the same stability on a lighter mount and the optics on the one I had were of very high quality. Great views. I have owned refractors from 80mm up to Astro Physics 6" and if I ever owned another refractor, it would be another 120ED. Not to big, not to small, and excellent performance. Refractors though are not really my thing (to slow) but the 120ED was (to me) a great compromise between size/weight and performance.