From what I know/heard both give you the same amount of replayability. That is to say none.
If you're a completionist you'll have to run through Far Cry 5 again on an insane or something like that mode. The Vietnam DLC you have to do 3 times different ways, Mars one is once through if you complete everything else before the final mission. I haven't messed about with the zombie one but think it's like 7 separate mini levels.
New Dawn I just bought and finished in the past month. I wasn't an early adopter because it just seemed like a Blood Dragon where it was pretty obvious the game was an idea board DLC that they knew they could milk some money out of so turned it into it's own thing. I wish I'd played them back to back now as it brings you back to a lot of locations from 5.
RDR2 I've not played and probably won't til well into next year. No single player DLC will be released and people either love or hate RDO there doesn't seem to be much middle ground opinion of it.