XPS 8930, SSD M.2 mount vs PCIe x4 slot

Anyone know why Dell charges a little less for an SSD using a PCIe slot and an adapter card than the same SSD going into the M.2 port on the motherboard? The SSD are the same capacity in both cases and otherwise typical x4 PCIe/NVMe devices. Aside from an x4 slot mounted SSD possibly staying a little cooler than a motherboard M.2 SSD are there other reasons for choosing one over the other?