Aparently you may only be able to use 2gig of your microdrive after modding the m3... for more info and user based discussion on M3 and its quirks (yes, there are still some problems with it, other than its size), I think there is even a thread there about using it with a microdrive too.
http://www.m3-forum.com/I think there is even some info on PassKey/PassKey2 etc there.
Be warned though, while Magic Key 2/3 will work fine as a passme, all current releases of the boot menu will not run on an M3 without modification (I have not tested the menu with pepsimans poweron thing yet).
PassKey 2 is for new firmware DS's (actually, its compatible with all DS's) and PassKey 1 only works on old firmware DS's. PK2 is harder to setup initially, so if you dont need it then dont get it.
There should be enough info on this forum and the M3 unofficial forum for you to make an informed decision and get the most out of your money.
I personally have a M3-SD, SC-SD, NeoMax, MK2/MK3 (all of them) and have to say at this point the most bang for the buck goes to supercard SD (it is cheap and the software is stable right now, at least for DS things), but I pin most of my hopes/expectations, and often look for updates to the MK 2/3 software, since I know it will eventually do alot more than it currently does, and offers things that the others dont (rom dumping, save emulation without using GBA cart SRAM, transparent patching, on the fly cart burning).
Each device has its plus and minus list, and depending on what you are using it for (cough *piracy versus development*) it is up to you to figure out what you need to do what you want.