before you even get into how valuable the DS's back catalogue of games and backwards compatibility in new models is I dont think downloadable games is anywhere near mums going into shops and buying their kids a copy on Nintendogs on the spur of the moment or for presents etc just yet.
I remember being in Woolworths buying GTA4 on launch day and a woman in front of me had a pink DS and 3 copies of Nintendogs - Apple cant compete with that.
There's no point in thinking they both have touch screens they're the same market in my opinion.