I've started buying E810s even for 10G links. PCIe Gen4, lower power draw, RDMA support, generally backward-compatible with SFP+ DACs and transceivers, and relatively inexpensive. Not nearly as dirt-cheap as the X520s but not crazy expensive (last I looked, at least). As I gradually replace switches over the next few years I can start taking advantage of 25G.

This is the way. The 10G cards are ancient and hog all the PCIe lanes.

I’ve had the best luck with Mellanox ConnectX 4 or 5 cards. The 5 can happily run 25G on a modern lane constrained system.