Replaced the rubber lines with stainless ones the last time I touched the brakes some 25+ years ago. Getting calipers rebuilt and replacing a couple hard lines (fitting rusted). Should I keep the stainless (they appear to be fine), get new ones, or go back to rubber?

Get new ones. I have stainless on mine right now but will change them in favor of rubber in the future.

Everyone seems to be shying away from the stainless because of abrasion issues I say buy the ones on sale

Are they the same between -4 and -6? I've learned to ask...
There is a set on classified for $50. Good price?

It’s good to change them more often than less since the rubber swells and narrows the id. Later rears on a four are different than early.