1. The difference is that a texture pack can only hold textures, while a resource pack can hold more than just textures like .ogg sound files and .json block/item model and blockstates files. (and language files, but who would edit or add new languages?)(Me!)
Back then in 1.5, it wasn't popular to say resource pack yet, because it could only hold textures and sound files, only at 1.8 did the change happen with the addition of item/block model and blockstates files.
2. Why make an overlay out of one? Why not combine them into a personal pack? Take the best from both worlds. (without the ability to ever publish it)