They are not in the same business, and I say this as a long-time PVG/PVR holder.
Both NRP and PVR started out as coal royalty MLPs. NRP branched out into timber and aggregates, but still kept the royalty business model. It owns the properties, and someone else operates the business. Coal is still the vast majority of the operation.
PVR moved into natural gas midstream several years ago, and natural gas now makes up a really big piece of the operation. Nat gas midstream makes up about 80% of revenue (unfair comparison, because PVR operates the midstream business, so its revenues are much higher than the coal royalties, but nowhere near as profitable as the coal.) Unfortunately, PVR hasn't figured out how to make the midstream business very profitable so far. They're trying to go for a more fee-based business, and they are expanding into the Marcellus, but so far their midstream business has pretty much sucked, to use a technical oil & gas term.
I started to move from PVR to NRP about two years ago, and after the merger with PVG, I'm moving more over here. NRP is better than PVR on the coal side (better growth), and there are lots of better midstream MLPs than PVR.
I think PVR is a good company but the fracking issue remains unresolved and the indictment as to its environmental impact is very strong. I'd be wary of much exposure to shale fracking( i.e. Marcellus in the case of PVR) until this is clarified.