The DQ6 has better cooling indeed. It covers both SB and NB, and the heatpipes don't block one of the PCI-E x1 slots like they do in the DS4. More importantly IMO, the DQ6 has 8 SATA ports, while the DS4 only has 6 ports. Also, the DQ6 comes with eSATA adapters in the box.
TBH I have no idea if these differences are worth the $70 difference. I'm using 6 SATA ports now but I don't think I'll need a 7th soon. I don't use any PCI-E x1 slot either. If it's just for eSATA, that can be added for much less than $70, closer to $10.