Here's a more detailed look.
I'm assuming you're choosing from the iPlayer, the DSTWO, and the EX4i.
Features that the DSTWO has that is missing from the EX4i:
-GBA speed settings (fastforward/rewind/frameskip, etc)
-GBA savestates
-GBA cheats
-DS Free Cheat
-DS Real Time Save
-DS Real Time Guide
-DS Slow Motion
-In-menu file management (Copy, Paste, Delete)
-Built-in AP detection
-Possibly more
Features that the DSTWO has that is missing from the iPlayer:
-GBA Emulator updates
-DS ROM loading (Commercial games)
-All the stuff that relates to DS ROM loading (excluding homebrew support)
-Possibly more
Features that the DSTWO promised but doesn't have yet:
-SNES Emulator
-.jpg file support in real time guide
-SDK (although this will be released after the SNES emulator is re-coded)
(No, the SC team did not promise a movie player)