I would say if you already have the D7100, it's not worth it to upgrade. The D7100 is already a very good camera. If you have neither, and can afford it, why not? Go for the D7200. The upgrades are minor, but they are upgrades. It does have a larger buffer, but it also has a newer, and (allegedly) better processor. Could you tell the difference between the two? Who knows. I got the D7200, because that is the one I wanted, and I'm happy with it. It's a long term keeper.
I would think a "casual photographer" would find it hard to beat a D7100 refurb vs a D7200 in particular.