SNEEK: the original version. You won't be able to access your SD card, since that is where the Wii is told that the NAND resides.
UNEEK: The only difference is that the SD card is now accessible. The NAND resides on a USB drive instead of the SD card. I should note that I have not been able to successfully launch much homebrew or any of my VC/WiiWare titles stored on my SD. I still prefer the NAND to be stored on the USB drive, though.
SNEEK+DI: Structured the same as SNEEK (so no SD card access), but the DI module allows for game loading from a USB drive. The reason that this is amazing is because the game will appear in the disc channel, and for all intents and purposes, the Wii thinks it's an actual disc. Games that suffer from very annoying compatibillity issues like CoD3, Red Steel, Sam & Max (season 1, but still not season 2. it's an oddball game , it seems), Monster Hunter Tri, Metroid Prime: Trilogy, etc. work without issue or without fiddling with settings.
UNEEK+DI: Structured the same as UNEEK, but now with game-loading capabilities.