What are the advantages of a 21° v 30° framing nailer?
None, assuming you're not in a hurricane or seismic zone or other area where their use is prohibited. They're just proprietary standards. Some manufacturers provide both to accommodate your existing tool set and nail supply, and so they can sell nails to owners of other gun brands. It's mostly a matter of a different magazine, so it's cheap to do so.

I've fired tens of thousands of both and never detected a practical difference in performance. Early guns used full-head nails probably because D-head hadn't been invented yet. The D-head design was initially to get more nails in a "stick"--they don't require spacing with wax or wire--and to allow the use of paper as a cheap binder.

D-head nails are still more likely to pull out of the secondary member than they are to pull through the primary member (though the initial bite of the adhesive coating makes pull-through possible in modern fast-grown lumber, which is softer). 

Use whichever gun seems most appropriate, and consider cost and availability of nails in your area. Also consider what your cohorts are using. Nail-sharing and organization can be a factor.