Big questions Heather
As you've already discovered, so much of the decision is based upon personal preference. I use strategies, as I simply want to be able to understand both
I definitely lean towards high ISO, wide aperture as my personal preference. I sometimes find the flash from my speedlight (external flash) just annoying to me and the people I'm shooting.
That being said, I am working on a tutorial for using external flash
I hope to have it available soon!
Your flash settings are so dependent upon the situation, distance from the subject, and where you are bouncing your flash (always bounce!).
As a rule of thumb I ALWAYS shoot in Manual when using the flash. It gives you more control over the light your camera records.
Laura posted an EXCELLENT explanation of using your external flash that I'm going to link you to. No reason to repeat the goodness she's already shared! You will need to join the DD POTD group to view it.
LauraBean's How I Use The Flash