Looks like rya is a triple store on top of accumulo. Jena has SDB (stores data in relational DB), TDB (single machine, single JVM, transaction-wise, on-disk store [with mmap, journal, cache, etc]).

Jena API is also extensible, and I remember someone was talking in the mailing list about an example with HBase I believe, as the backend for the triple store.

Both HBase and Accumulo can be scaled to multiple nodes. So you should have some pros and cons of using either (or rya), as well as pros and cons of using Jena.

Finally, I believe Jena is not simply a triple store. It has a web layer via Fuseki, services for managing graphs, command lines, and other tools useful for data processing with semantic web technologies.