RAM and SSD are two entirely different components. What are you asking?
RAM in the 2018 Mini is 2666MHz and can be upgraded, Crucial has kits and the speed of memory is dictated by the memory bus on the motherboard. If you intend to upgrade the memory, any compatible memory will be roughly the same speed.
SSD is not upgradable, it comes soldered to the motherboard and is PCIe.
It doesn't matter. It's off the shelf stuff. The few Mhz difference will be inconceivable in the real world. It might show up in benchmarks, but honestly who cares. Get the one at the best price and spend the left-over cash on beer.