Depends on your use case. I believe the Banana Pi Pro is supported by Debian Jessie with the stock kernel if you can do without video and audio out and a handful of other features you're unlikely to need in a headless server. See http://linux-sunxi.org/Linux_mainlining_effort - Banana Pi /Pro is A20, Jessie ships Linux 3.16. (Audio and unaccelerated graphics are supported by newer mainline kernels, in theory at least.)

The ODROID-C2 (their AArch64 offering) was added to the Linux kernel device tree in March: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux....

But Hardkernel does seem to have a lot of additional patches in the kernel that they ship: https://github.com/hardkernel/linux/tree/odroidc2-3.14.y