The TS-259 Pro is the more appropriate equivalent to the DX710+, as both have the dual core Atom D510. The TS-239 Pro II is only single core Atom D410.
The Synology is expandable, which is nice, but the DX510 is expensive and consumer the 1 eSATA port. Plus, you are now driving multiple drives through 1 SATA interface, which has performance impacts. The QNAP unit is not expandable, but has 2 eSATA ports and 5 USB, versus 1 eSATA/3 USB on the Synology. The QNAP has dual GigE NICs vs 1 in the Synology.
Synology seems to have a little more overall polish on their bundled apps, but QNAP has been working on that. QNAP is all metal, Synology is plastic. QNAP already has an iPhone/iPod Touch app, and just released an Android beta. Synology is supposed to release one with DSM 2.4.
I'm not sure who offers better camera support to be honest. QNAP specifically sells products for camera use, so they may put a little more emphasis into their camera piece of the firmware.