Play-yan won't run in the DS's res,
so you sacerficing resolution for a boost in picture quallity (not a trade off i'd take).
http://www.lik-sang.com/info.php?cat...;all_reviews=1 <- very bottom has some links that show you what to expect in terms of the "GBA movie advance" quallity.
The M3 will play videos encoded in the same format, but in higher res...
Don't forget, that the M3 gives you the abillity too emulate a TON of games (NES, SNES, Gameboy, GBC, etc etc)..
You can also play homebrew apps (ie DSLinux,)