Yep, DOT 3, 4, 5.1 are hygroscopic, some act like that is a bad thing? In the past oil based was used, living on the water planet, it's unavoidable. In that type of fluid, water stays as droplets, puddles in low spots, like the calipers, making it very hard to remove. When heated, at about 212F, it turns to vapor, instant soft or no pedal. The change to hygroscopic was because moisture is carried in the fluid, slowly reducing the boiling point and making it much easier to flush out. So it's a good thing, but we are back to fresh fluid is better than old fluid, all of the types require maintenance.
Is 5.1 better than 3, yep it says so right on the bottle! When you go the the gas pump, it says right on the sticker that premium is better. If you motor isn't tuned to take advantage of higher octane fuel, you are just wasting cash, no other benefit. The '80 brakes do a good job of dissipating heat, high void area rotors, etc, DOT3 is spec for it, decades of abuse have shown that it works well. If we were talking stock cars at Martinsville, it would be different!
Any of it will work, run what you like, but for best performance and part life, keep it fresh. Personally I run the cheap stuff. Suck the reservoir dry and refill a couple of times a year and fully flush at each brake job is good practice. Clean it up before it looks like this!