beta - issues specific to SE 2.0 sites that have been created in Area 51 and are currently in the Beta phase.
beta-sites - This tag is used for posts that are related to those Stack Exchange sites which are currently in beta phase. Beta sites have a different website theme as compared to full member (graduate) sites of the Stack Exchange Network. Reputation points requirements for some privileges are different.
I can't see that there is any real difference between them though.