The mainline images using the mainline kernel - the kernel provided by Debian. Non-mainline use the vendor kernel (from Radxa, which get the base from Rockchip) which probably has some extra patches and other optimizations and changes.
For maintainability the mainline image is easier for us, but maybe some specific functions of your SBC will not work correctly. For the vendor image it’s the opposite.
@MichaIng can provide a more detailed answer if you want.