Just on the batter side.

It might be a tad better to have them on both sides but it isn't worth the hassle in case you have to tune them because of a change of humidity.

You just can't reach to the reso head while playing a gig.

But I haven't really tried calf on both sides even in controlled environments like studios.

So I can't really tell if it is that much of a difference.

Griener is right on the money. Calf for batter, modern (Aquarian MV for me) on the reso side. In a climate-controlled environ calf on both sides is fantastic. I've had good luck buying used calf heads off eBay or CL. Don't worry about how they look - just make sure there are no rips. You will love the stick response of calf.