The multi-touch interface is horrible at scrolling through long lists. Short lists, it's great; superior to the click-wheel. I mean, to each his own, but I've used a friend's extensively and it's one of the major reasons I waffled and probably won't buy one. For a small amount of music on a 4GB or 8GB, sure. But even with a lot of music on his 8GB model, it was a pain; I couldn't wait to get back to the easy, fast scrolling of the click-wheel.
And how can you compare, really? Even if you devote all the usable space on an 8GB to music, it can barely hold the number of songs as my 30GB video iPod can hold albums. I think it's a great interface for the iPhone. I think the mouse is a great interface for a computer, but I wouldn't want to use it to control a toaster.
Multi-touch would be fantastic for large collections, IMO. They just need to provide the same 'alphabet list' that they have on the iPhone's contacts list, so that you can zoom through your collection by letter. No-brainer.
Or, hell, an active search function, which they already have on current iPods, so I can't see being an issue with the rumored multi-touch iPod.