I don't play a lot of PE, because the controls are a bit too awkward for me to get used to, but I've been pretty interested in the modding scene of the version for a while, because it mirrors the way Minecraft used to be in the early days when it came to modding in a lot of ways. Just a lot of experimentation to see what's possible.
I'd say... pretty far. A PC's graphics capabilities are WAY more advanced than a phone's. There are more ways to change gameplay on the pc version without much trouble. Minecraft PE, has been a platform not a lot of people have focused modding on and the only way to install mods is the stupid BlockLauncher App which works, fine, I guess... When add-ons were added, modding became easier, because BlockLauncher has its limits.
PC Modding basically has infinite possibilities.
PE Modding has the problem of actually 'adding' blocks, mobs, and items, something that add-ons and BlockLauncher have never truly helped us coders achieve. (it just doesn't feel as real as the PC version)
And another problem I'd say is trying to add models. There are literally NO model makers for pocket edition so you could design mobs and their shape and textures. PC version has a LOT of model makers which easily allow you to create custom models and their textures. Just click Create and poof! There you have your code to use for the model!
So basically, PC version can have models with better graphics, has a way to add/remove EVERYTHING using mods, but PE (now called Bedrock Edition) can't do much. And BlockLauncher is glitchy and never truly worked for everybody, so every mod maker out there gets messages from random people saying the game crashing for their mod.
Still, I wouldn't discourage you from making some mods for PE. There are actually only a few mods that are actually working properly for PE, most of them being add-ons. Right now, lots of people are searching for PE mods but can't find good ones. The modding community is pretty new and basic, so maybe you might be the first one to utilize advanced coding and figure out a better way for players to install mods or something. We would definitely appreciate new modders at this particular time. Welcome to the modding community, dogman035. If you need help in modding, PM me and I'll see what I can do.
But if you work hard enough, and long enough, you can make pretty good mods on both versions.
If you put your heart, mind, and soul to anything, the limit is the sky.
I'm not really that experienced on the matter, so I suggest you take help from others as well.