Soldering question: Which Kester solder to use (Resin-Five vs. SN63) in what situation
The Resin-Five solder seems to melt at a lower temperature and flow
better, but the finer Solid Wire solder looks cleaner like it is more
appropriate for PCB soldering of small digital components.

This matches my own preferences. Large diameter 60/40 rosin-cored solder for general use, finer diameter for small surface mount parts (with external flux from a 'flux pen'). For very fine work I sometimes use 0.7 mm.
SN63/PB37 'eutectic' solder is supposed to be usable at a lower temperature and produce brighter joints than 60/40, but in practice I haven't found much difference between them.

I also have several choices of soldering irons ranging from 20-40 watt
pencils to a bigger dual-power Weller soldering gun. I have used the
smaller irons on PCB components and the larger gun for larger line
voltage wiring.

Again I think you have the right idea. I use a 40 watt temperature controlled soldering station for most PCB work, and a Goot TQ-77 20/200W 'gun' style iron for soldering heavy wires and connectors.