This is such a tough question because, IMO, the answer will vary depending on each project. Sometimes you might want a small circle and a much bigger scalloped circle. Sometimes you might want a scalloped circle and a slightly larger regular circle. I think many different variations will all look balanced, depending on the paper you use and the specific application of each circle.
Do you have a machine like a Big Shot or Cuttlebug? If so, then you can get greater variation with Nesties than you ever will with two punches. Of course, this is a pricier option However, my experience with circle punches is that I always want another size and the cost of these adds up a lot, so a machine with Nesties might be cheaper in the end, if you can do it.