It depends really. What size memory would your iTouch have? The 32GB and 64GB ones are faster than the other ones IIRC. I'd go for a Zune, but that's because I don't like Apple. My suggestion would be to get both, in this manner:
Nintendo DS Lite
iPod Nano
Everyone's a winner - you get an MP3 Player, and pretty much the same console. Plus, you can play GBA games on the Lite, unlike the DSi (someone corrent me if I'm wrong).