You're missing the most realistic middle ground -- Sanchez starts because Geno isn't ready. Sanchez gets us through the tough part of the schedule, then Geno takes over to get experience against the lesser competition.
Neither case has much to do with who is "better" as both are likely to struggle. It just makes sense to let Mark take the lumps early then finish the season seeing what we have in Smith before we head into '14 . . .