I have no general interest in what OS comes pre-installed on any eMMC module. Having a pre-installed OS will come in handy for someone just starting out. It's the eMMC module itself that will be very important to you later and the eMMC module you are paying for. You can format it and use it again many times.
I try to use an eMMC module for my OS on my C2 and XU4 SBCs. If I use an SD card at all, it's either to flash some particular OS from the SD card to the eMMC module, added storage. The only exception to this is when an image I want to run exceeds the highest capacity eMMC module available.
There are SD cards available with very fast advertised read / write speeds. Many can reach the claimed speeds in the right USB 3.1 adapter.
Buying the fastest SD card available will be limited to the speed of the SBC's card slot. With any SBC that can use eMMC or SD, the read / write speeds will always be faster with an eMMC module. eMMC has a big performance advantage over SD cards and they will last a lot longer than an SD card. I think many people have a hard time justifying the initial cost. The 128GB model costs considerably more than the XU4 SBC itself. Buy one and compare for yourself. Once you realize the benefits of an eMMC module, you will realize its money well spent.
They really are worth every penny.