There are at least 3 non-interchangable versions to speak of...
Most Model 1 consoles (in my experience) have a 3.3v drive, which might be from a couple of different manufacturers. These should be all interchangeable as far as I know (just make sure the drive actually says "3.3V" on the metal part on the bottom right corner -- it's in fairly large print, see the picture below).
There are also consoles (model 0 and some model 1, possibly?) that have 5V drives. These look pretty much the same as the 3.3v ones, but they are not interchangeable. I don't recall if these say 5V on them or not, because it's been a while since I opened up my model 0 console...
Finally, some later consoles (Model 2 ones) have a much lighter GD-ROM module. These drives do not have an on-board controller like the other models because the GD-ROM's controller hardware was integrated into the motherboard of these consoles. These are also non-interchangeable with any other model of the system, quite obviously.
I'll try to grab pictures of the 5V drive and the later drive sometime, if I can find them in my collection of consoles...