How Do You Decide On What To Collect?

Well, I think that we really don't decide. That is, we don't (or can't) rationally weigh the pros and cons of collecting different things. It may just be a matter of opportunity, convenience, cost, or some random factor like a book or a lecture, assuming that we already have the collecting mentality. There may be something in your brain that finds something of interest from an esthetic or historical perspective and you go for that. Most of the posts here are dealing with American coinage, but the same thing applies to everything numismatic. The field of American exonumia alone is huge with many categories. If you add a worldwide scope it just opens up the possibilities even more.

As an example, one of my sub-collecting fields is exonumia from my city of Ottawa Canada. I started this a few years ago and I have 150+ items now. I certainly would not be doing this if I didn't live here, and as you can imagine, exonumia tends to be concentrated in the area of origin.