The 2 bay Qnap TS-253D vs 4 bay, worth the PCIe 4x4 future upgrade with 2 bay?

[ashkasto]

I am looking at the Qnap TS-253D model, My understanding is the 2-bay offers a PCIe which is 4x4 speed for future upgrade verses the 4-bay or 6 bay offers a 2x4 speed PCIe speed. How much of a difference in performance would that be for adding NVMe drives for cache or 10GBe? Am I better off getting 2bay and getting expansion bays to make use of the 4x4 speed?

Thanks