9mm gives you a lot more bullet choices, across a different weight spread, with higher velocity. Also shortens up the length of the reload (9mm in moonclips are shorter than .38 in moonclips). Recoil is subjective, but I noticed no noticeable difference between.38 +P and standard pressure 9mm.
In regards to 9mm in a .38/.357 barrel, there are usually no issues. I kept my .38 cylinder for my 642-1, and don’t notice any noticeable difference in accuracy. I read the Ruger convertible threads, which both pushed me towards a .45 New Vaquero Convertible and made me hesitate with the 9mm conversion. Results of the latter really make the Ruger topics come into question, in my eyes... because I shouldn’t have the results if there wasn’t some type of other factor involved. Don’t know what it is, but can confidently say you won’t have issues with a 9mm conversion... if you don’t abuse the gun.