This has always bugged me; a long time ago (like, 15 years), I spent some hours trawling through books 1-6 or so looking for evidence that there is a substantive difference other than "a sa'angreal is a much more powerful angreal", or as the glossary put it, "it increases the amount of Power that can be drawn as much more than an angreal does, than an angreal does over channeling unaided" (or something close to that).
There may be partial hints later in the series. The way the Choedan Kal is described repeatedly makes it sound as though it provides a fixed, enormous amount of channeling capacity. Is the difference perhaps that an angreal provides a small multiplier? Was this answered in any interviews or RPG manuals that I haven't seen?