I'm waiting on some hardware to arrive but basically I have an ASUS RT-N16 router running DD-WRT and I want to have mysql(for xbmc library), sickbeard, and transmission running on an Rpi as its primary duty. I'm going to have an Ouya running XBMC which will need to access the drive over the network. Is there a clear "better" choice between putting my USB harddrive on my router and having it do the NFS/NAS serving or putting the HD on the Rpi and having it do the NAS/NFS work? My intuition, which is often bad for these sorts of questions, says that it probably won't make a noticeable difference which box runs the NAS but figured I'd ask.

Here are some more details in case they make a difference.

I plan on running OpenVPN for all internet connections from the Rpi.
I don't use the router as VPN client, only each device runs VPN if necessary.