I wouldn't blame Paul & Mary for staying with the show. Why should they give up a show they clearly love doing just because it has moved channels? Regardless of the pay packet (which I'm sure they'll consider, they'd be idiots not to), if they enjoy doing it then why not carry on?
Plus, surely they'd just be depriving millions of fans of the show they love? Mel & Sue already have done in order to make a statement, which was all very noble of them but ultimately whether they agree with what's happened or not, the show could have continued in pretty much the exact same form for the millions of fans it has, had they not left. For that reason I wouldn't blame Paul & Mary for moving with it.
I can see the arguments for not moving and taking a BBC deal as well, but I don't think it makes them awful people (as some on here seem to be making out) if they decide to stay with the show.