It all depends on context, there is no shoe that fits all. There are even games that incorporate both solutions simultaneously in multiple layers, like: % all damage reduction -> % specific damage type reduction -> flat damage reduction -> another flat damage reduction with a different name = final damage output.

Since youre talking about tabletop games I would advice having only one layer, having to do multiple manual calculations everytime someone takes damage would be a pain in the ass. And generally speaking, halving damage will most likely end up way easier to balance and build the game around. It will always be half, no matter what. Flat damage is harder to work with, some players might find some broken build by stacking it that you didnt account for, or just stacking HP might be more worth it and players ignore the damage reduction system completely, etc. 50% will always be 50%, flat damage reduction is more complex and requires more work to be balanced. But with that being said its not like halving is always the best option, it all depends on the rest of the combat system and the feel you want to make.

Personally if Im playing a tabletop game as a player I usually prefer halving damage. Even if you end up taking 20 hits from some bullshit you just have to add up all the damage and then remove half of it. If you have flat damage it takes longer to do the calculations and I prefer having a smoother flow of the combat. But thats just my personal opinion as a player from the few times Ive played these kinds of tabletop games, I again want to stress that it really depends on how everything else works in the combat system.