One of the things a lot of games differ on is whether or not the target of an attack makes a skill check to defend themselves against the attack, or if the attacker just makes a roll against a flat difficulty that is arbitrarily assigned by some combination of the target's relevant stats. There are pros and cons to both, including some personal bias on the part of the people who make the games, but I wanted to talk about that today.
The Core Argument
The core argument I've seen people make in regards to which one is better has essentially come down to 'time' versus 'agency.' Now, Time is on the side of the set target number for being hit, and the reason is essentially that it reduces the number of dice rolls during combat, which in turn reduces the amount of time a round of combat takes. Agency, on the other hand, sides with the target making a defense roll. The argument essentially states: we don't just set some number for how good we are at attacking, so why do we do we do it for defense?
Now, obviously there is more to both than just that, but those do seem to be the core arguments, and thus will be where my own discussion focuses.
Set Target Number
The Set Target Number type of defense is the classic and is as old as, well, as RPGs in general. AD&D had its system where the better armored/protected you were, the lower your Armor Class was. Third ed, and I believe 4th ed, adds your armor, dex, and other bonuses onto a base of 10 to set your armor class for that. Other games use this too, and it's not at all uncommon.
As said, the argument for this is that it reduces time needed for combat by reducing rolls. Since you aren't making 2 rolls to see if an attack hits, you don't have as much to calculate and things go faster. This is further argued by the fact that defending oneself is often reflexive, and not as active as attacking another person, and so you just (for D&D's sake) have them take a 10 on their defense check.
However, one of the arguments against the armor class rule points out that by taking a 10, you are losing out on the 50% of the time that you would be harder to hit. In a game focused on combat, that is a big number to think about. If your AC is 18, and you got hit by someone rolling an 18 on the nose, then - had you rolled your defense - 50% of the time you would not have been hit or be taking that damage. Of course, on the other hand for that is the fact that 45% of the time your AC would be worse (and 5% it would be the same.)
Roll Your Defense
The other side of the argument is for letting people roll their defense. Among other things, this gives a sense of agency to the player in when they get hit, but it also opens the door up to other things happening - which some claim is more 'realistic.'
Now, the sense of agency comes from the fact that when you were hit, it is because you were out rolled. Sure, maybe you don't have much say in how that die comes out, but you at least did something. It just happened to not work this time. I'm not sure how fully I endorse this argument now, but it was one I enthusiastically supported earlier in my life and I can see it still.
No, my personal preference in rolled defense systems is the variety of outcomes it allows to happen. Sometimes people just mess up their defense. You go left when you should've gone right, opened up when you should've hunkered down, or just stood and absorbed when you should've moved to one side. The point is, sometimes people mess up their defense, and sometimes they do an amazing job of covering up. The rolled defense allows that to happen. It also allows status effects to come into play with certain bad types of defending. The D20 Game of Thrones RPG had a system where - depending on your armor - a bad roll would cause fatigue. Fatigue would give a -1 penalty to all checks, and the only way to avoid rolling was for an attack to be so low that you wouldn't have had to roll to defend against it at all. This added a level of strategy to outfitting your character, because while Plate Mail added a nice hefty +8 or so to your defense, it also caused fatigue on a natural 8 or less. So, 40% of the time you were fatigued when someone attacked you. A more lightly armored fighter on the other hand may only be getting fatigued 10 to 20% of the time.
The other reason I see for having rolled defense, is because we roll both dice every other time characters are in contest. If a thief is trying to sneak past a guard, we don't add 10 to the guard's Notice and have the thief roll against it. We roll the guard's notice versus the thief's stealth, and see who won. A single die roll isn't, in most systems, a huge deal, and with a properly set up group it doesn't add much more time at all (just tell them to roll when you declare the attack. Rolls happen at the same time.)
Some systems do a combination. Perhaps the most unique of these is Icons. In Icons, whenever there is a contested roll between two characters the PC makes the check. So, if a PC attacks an NPC, they roll a check against the NPC's defense to see if they get hit. If a NPC attacks a PC, the PC rolls their defense against the NPC's attack to see if they can dodge. This means the DM never has to roll dice, and the player is always determining their own outcome.
Other systems do different hybrids. L5R lets a player who is focusing on their defense make a skill roll to set their Target Number to be Hit, for example. D&D can also easily be converted either way, since the defense rules state that by default it just assumes the player took a 10 on their defense.
Thinking about it now, I think I would like to see a permutation of D&D (or some other game) where a character can focus on their attack or their defense. If focused on attack, their defense is 10+modifiers. If focused on defense, their attack is 10+modifiers. You could either split this by class, or you could just have it be a round by round decision the player makes. I could see fighters focusing more on their defense as their default attack is good enough to just always hit, or swapping back and forth depending on their needs for the round. I think it might be interesting.
How about you? Do you have a preference between the two? Depending on theme of game, I like both for the reasons I've stated above.