Having bought some X520-SR2 cards recently I learned that they require a PCI-e (2.0 or newer) x8 slot to run both network ports at full bandwidth. You can plug it in to an x4 slot and it'll work, but full performance will only be available to one port at a time; the other will be capped at something less.
The X710-DA2, on the other hand, supports full bandwidth on both ports in a PCI-e (3.0 or newer) x4 slot. It is an x8 card (just like the X520-SR2), but the extra lanes are present only to support full bandwidth on both ports when attached to PCI-e 2.0.
This mattered to me because my newer motherboards only had (free) x4 slots.
