It's 1/128 from the first male, not 1/28 technically, to produce the exact combo of Batman. The odds of producing Batman/Batman+ are exactly the same, 1/8. That's why it's really important to really understand what's behind the odds rather than just pop things in the a calculator.
If it were me, I'd go for the GHI male, see if I could hit world's first, though it's a long shot even if nobody has done it. GHI is so amazing, and GHI Batman has got to be amazing. Banana is a great morph, but IMHO Banana Batman is a step backwards from Batman