Selfhosted

My current setup is two always-on hard drives hooked up to this two-bay external hard drive docking station plugged into a laptop via USB cable for whatever network sharing I may need. This has been good enough so far, but I'm anticipating the need to expand down the road, i.e. adding a third drive. When that time comes, part of me thinks I oughta just spring for the 4-bay version of what I already have and keep on keeping on. Another part of me thinks maybe I should plan a new build in a mid-ATX case w/ 4 or 5 HDD mounting slots for future expandability.

One thing about the external docking station that appeals to me is how portable it is, meaning if I ever want to spring for a beefier laptop or one of those thin clients, I can just plug the HDD docking station into the new host and away I go. Another nice thing about laptops and SFF equipment is how energy efficient they are. On the other hand, planning a new ATX build w/ HDDs mounted internally would enable me to plan the whole thing top to bottom w/ whichever components I like, but I'm a tad concerned about how feasible it is to achieve the same level of power efficiency w/ an ATX build compared to a laptop/SFF/thin client w/ external docking station.

Has anyone else out there had this dilemma, and which way did you go? Any advice or warnings about what might come back to bite me down the road if I stick w/ the external docking station or go w/ an ATX build?