I'm not an expert on this subject, but Whitepost is, so when I redid my 40 Dodge brakes (they've sleeved all my cylinders on 3 vehicles) I asked them these questions. This may start a war, but what they told me was that dot 3 absorbs water, dot 5 does not. Absolutely do not use dot 5, as the water collects separately and causes corrosion. Dot 3 absorbs the water and keeps it from causing corrosion. There is a limit to the water it can absorb, so you must flush it occasionally. I don't recall them giving a time limit, but I do mine about every 3 years.
the advantage of dot 5 is it withstands much higher temperatures. So, i'd say if you are road racing your vintage Mopar and are anticipating the brake fluid boiling from heat, dot 5 is your answer. But flush it often.
I have a motorcycle that had dot 5 from the factory, it's intended for hard use. When I got it, as a normal maintenance thing, I flushed the brakes. I got fluid and water and a little trash. Very seperated. I have never seen that when flushing my Mopars which had dot 3.
if you have questions, call Whitepost and chat with them. I'm happy with my results, I'll stay out of any ensuing war.