Get a flip-flop hub on your new bike. that'll give you two options. Slap a Dingle on one side, now you've got three. Add a Dingle to the other side, now you've got four.

My personal favorite setup is fixed/free. I do like to ride in the country and have been known to go just a little too far. Having ss on one side is great when you are tired.

Others like to have a racing cog and a tooling around town cog. Or one low for tricks, one high for speed.