Yes, the 5'10" would perform less well on bigger days. But the reality is the Potatonator will perform less well on bigger days regardless of the size you choose. It's a small to medium wave board, emphasising small medium over large medium. If you want more of an all rounder but still capable of surfing small days maybe look at the Spitfire or Dom instead. If you're happy with a board for small to medium days (that's the Potatonator's intended niche) then get the Pot and 5'8" to 5'10" will both work.