Either one would have suited by needs at home but cost was the biggest driving factor for using the X520-DA2.
For the office on the other hand, I was inclined to run some side-by-side tests between Mellanox, Solarflare, and Intel 40G cards.
I seem to have found an answer as to why the XL710-QDA2 adapters aren't as widely available.
See the October 22nd revision to the XL710 controller spec:
48. XL710 40 GbE MAC Transmit Packet Issue (Updated - Fixed in B1 Stepping)
In the XL710 40 GbE MAC, if the packet to be transmitted contains a specific 8-byte sequence within the header or payload, an invalid packet is transmitted and results in a CRC error at the receiver.
This issue impacts all 40 GbE modes that the XL710 supports.