There are absolutely meaningful differences between the parties, but the mainstream, and who we end up having to choose between, trend to the center, which is where the similarities are. I think a well-informed citizen must keep in mind the ways both parties rally to particular issues and can be hostile to common interests while differing, sometimes in huge and significant ways, on other things. You're right that the "they're the same thing" is blithe and shallow, but that doesn't mean there aren't similarities.