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Both of these are tough calls, and everyone here has made valid points. Personally, I see why Int makes sense for a Witch, but Wis or Cha would, I imagine, be equally logical.
I don't really see Charisma for the Summoner making as much sense, but it depends on the character--does he have a natural connection to this extraplanar creature, or is it something he studied and researched? Probably could be either, but my knee-jerk reaction is the latter, so maybe the Summoner should be Int.
This is related and has been touched on, but never explicitly discussed, so I'm going to go there--I don't see, off the top of my head, why the Summoner should even be a spontaneous caster. Their main magic focus is their Eidolon, and the process of summoning, augmenting, and controlling this extraplanar creature seems more "wizard" than "sorceror" to me. It seems logical to make them prepare their spells in advance like a wizard, without hearing some justification for why a Summoner should cast like a sorceror or bard. At least at my gaming table, being a spontaneous caster has always been considered a huge boon, and I'm more concerned with everyone and their brother not preparing spells than I am with how many classes are or aren't sharing the same prime requisite (pragmatically speaking, what difference does that really make anyway?).
Oddly enough, given my previous paragraph, there's a case to be made that a witch shouldn't have to prepare her spells, though the FamiliarBook gives an explanation for why it is the way it is. It may be a case of balance, since the witch has a pretty impressive spell list overall, without even counting the spares from the familiar.