I own both. Another Iriver vote is unnecessary at this point, but I don't know if all the advantages have been described.
The major plusses of the H320 IMO:
Rockbox is a MUCH better OS (I don't believe you can Rockbox the JB3)
Better battery life
Not prone to overheating (I've had several lost sets on sweltering days at outdoor festivals with the JB3)
Better meters
Smaller size
No need for proprietary software to be installed (handy if you need to hook up to someone else's computer)
The only disadvantage is the lack of optical input - otherwise it's a slam dunk...