It depends whether you're thinking about using a Maxie variant or a traditional evolution variant. A Maxie variant definitely will prefer Shaymin-EX, as Octillery lines are clunkier and take time. Evolution variants probably won't mind either Shaymin or Octillery, but would like Shaymin for a bigger early-game boost, and Octillery for consistency boost. It is possible to run a Shaymin-EX and 2-2 Octillery.
I doubt you'll have space, and anyways, I'd generally suggest not and instead go for some other option, as Bats are really only useful when you need to really increase your damage output (i.e. in Toad's case) or you're running a powerful sniping deck (i.e. Landorus/Bats). Bats will not work in a Maxie's variant because of clunk, and you won't have space in an evolution deck.