Was lurking...
I downloaded both packages for the Mac and gave them a quick try. For flowcharting (only) yEd looks pretty good - and for flowcharting plus a lot of other flavors of diagramming, Dia looks like a winner (although it uses up a sizable amount of file space).
I decided that for the time being I'd keep yEd for flowcharts, and see how much I actually need the capabilities of Dia. If it turns out that I don't need all that capability, I'll un-install Dia.
I think I'll retire my old green plastic template. 