Okay, this is an rather easy question.
Both are the exact same. In fact, I dare say it may be a bit more frustrating to mod 1.15 Minecraft. In the end, whichever version you choose is more of a matter which one you prefer.
First up, which version do you prefer. I believe it is better to play on a version you a familiar with. I usually play on 1.7.10 or 1.12 but that is a personal preference.
Second, are the mods up to date. Most mods only get updated when the me version comes out or after a month of two of release which makes modding later versions a bit more difficult. However, if you are using a Modpack, try to use the latest version of the modpack as the older modpack files will be for older versions.
Remember this and you should be good.
Requires:
Just dowload shaders from the marketplace or something. Why are you playing 1.12.2 anyway? Thats really old.