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.
Just dowload shaders from the marketplace or something. Why are you playing 1.12.2 anyway? Thats really old.