I think there are two important considerations. First, it depends on whether the items in your "groups of items" are wide or tall. Text items, for example, should almost always be listed horizontally, so that the distance between items (ie, the line height) is both relatively short (and, in the case of text, constant). For short lists, this isn't so important, but playlists are arbitrarily long. Compare this list:
All Along the Watchtower | SpottieOttieDopaliscious | Let's Dance | Watching the Detectives
To this one:
- All Along the Watchtower
- Let's Dance
- Watching the Detectives
OK, so the comparison here isn't true because I can't control the formatting of the list, but is illustrative of my points: the vertical list is easier to scan and (with equivalent formatting) makes better use of the display area (after just one more song, the horizontal list will bleed off the right edge).
Of course if you have items that are tall, you'll get more mileage out of listing them horizontally. If they're exactly square, it doesn't matter.
The second consideration is that horizontal scrolling is still a second class citizen on a number of platforms and, even, on supported platforms is a more recent paradigm. If you are designing for a platform and user base that supports horizontal scrolling equally well, then this point is moot, but I tend to prefer vertical scrolling in general for this very reason.