So much this and SE is to blame for giving us such a crappy way to get a house.
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I'd say go for the medium if you want the medium. FYI however there are more perks with bigger houses that do benefit FCs with multiple people. Bigger houses allow for additional crafting stations, more of the housing npcs and additional garden space (flower pots can't cross). It won't kill an FC not to have those things, but they are certainly nice things to have.