Models aren't really THAT complex. It's just a few ModelRenderers that have boxes set with a relative position and rotation. In my opinion, if you want to learn how to mod the game (i.e. add new models and such), then look at how Minecraft does it already. If you want your mod to be compatible with other mods, go with Forge, but if you are just learning and want to mess around with the game and aren't planning on releasing your mod, you can stick with editing base classes, but it is still better to learn Forge and stuff as well, as if you want to release mods later on, it will be easy