Now it depends of the use scenario. Old kernel is also not perfect, has bugs and ... it's old, general support is fading.
Mainline kernel is still considered development with H3 chips ... so I would not stand on neither side, while I would recommend mainline kernel for older Allwinner chips, A10 and A20. There, it's clearly a better choice in most use cases, instead of old stock kernel. Soon, here (H3, H5, A64, ...) it will be the same.