When using a clone stamp tool I spend a lot of time switching between a feathered brush set at 20% opacity and a hard edge brush set at 100% opacity. is there a way to set up a shortcut key for these type of brush presets, or even just easily switch between current and previously used brushes. I've seen a few trick/shortcuts, but it seems the difference in opacity between the two brushes will always have to be adjusted manually. Anyone know a trick?

I don't know about switching presets but in case that you may find this useful: you can change brush opacity using the number keys (1 for 10%, 2 for 20%, 37 for 37% etc) and for changing brush hard edge use Shift + [ or Shift + ]. In case you use a tablet you can even program the scroll strips for these.

You could record an action for each preset and in actions panel menu go to action options and assign one of the F keys for each action.
Then use autohotkey to script those F keys to any hotkey you want.
That's what I would do anyway.
Another option is to do it through scripting and then assign a hotkey to the script but I'm not sure if you can do that with brush presets. I tried only with tool presets.
But the problem with tool presets is that it saves the brush size too. so not an option.
Good Luck.