6800Pro - About 2x Cost of 4800+

I am interested in the 6800Pro for its extra two drive ports for expandability. With it being almost twice the cost of the 4800+, isn't buying another 4800+ (or whatever future similar model) be comparable if I need to expand? It will also give me 2 extra drive slots. Main NAS use is for file, video, and photo storage. What considerations am I missing?