360 runs both same as a wii, its only the 360 has more data in there games, look at lost odyssey for 1, 25GB highest wii will get is SSBB, which is a dual layer disc, so your question is answered, both take single and dual layer.
Once again, Smash Bros is the only DL game on Wii so far.
IIRC, all XBOX 360 games are on DLs regardless of size for security/anti-piracy reasons. That's what someone told me anyway (raises the cost of making backups). Of course the drive can read single-layer discs though (DVD playback, XBOX original games, etc).