One nice feature of the DSi is that you can go into the system settings and turn off wireless communication. I'm sure this saves some battery life during normal gameplay. Would be interesting to see that tested properly. If I ever have the time, I'll give it a shot.
Something like:
DSi, highest brightness setting, regular game, WC on
DSi, highest brightness setting, regular game, WC off
DSi, highest brightness setting, flashcart, WC on
DSi, highest brightness setting, flashcart, WC off
DSi, sleep mode, regular game
DSi, sleep mode, flashcart
Using same game & rom at intro screen, same volume levels.
Certainly wouldn't tell us anything about DSi vs DSL, but it might answer a few other questions.
Would probably take me a few days, but I might get started on it.