Were legality not an issue, a baton hands down. Especially if we are talking folders for knives.
As anything more than an absolute last ditch lethal force option, folding knives really, really, really suck. I don't care how tacticool or fast opening your folding knife is. There are serious problems with it. First and foremost is access. If you keep it in a jeans pocket, forget about it. You will not be able to get it out when you need it under stress. Oh, yes, I know you can stand there and practice reaching into your pocket and flicking out your knife, but the moment even a half serious training partner, let alone an attacker comes at you under stress, you will:
1. Not be able to locate your pocket, because your legs are moving
2. Not be able to get your hand or finger into your front pocket even if you locate it, because it closes up when you crouch down
Then, on the chance you are quite lucky and/or become quite skilled *with a particular pair of jeans* (everything changes when you put on a different pair with a slightly different cut or different pockets), you will inevitably fumble, grabbing the knife the wrong way, failing to open the blade, etc. etc. etc.
And even if you train like mad to do all of those things at once, and achieve a somewhat reasonable success rate deploying your weapon, you're still a whopping four steps behind your opponent. Hand in, Knife out, Open Knife, Counter Attack -- by which time he has made 4-5 thrusts or cuts at you. Good luck.
How do I know all of this? I trained like mad to make it work, and my solid conclusion is that, while I'm sure someone in the world out there can make it work for them successfully (one of my Kali teachers could), it's not at all a reliable option for the average martial artist -- not even the average dedicated martial artist.
Add to this two more factors:
1. Most folding knives with liner locks are apt to collapse or fold in on your hand, if not outright break, if you stab anything with significant resistance.
2. Many lack a grip that would keep your hand from riding up on the blade when thrusting.
3. Knives, while potentially quite lethal, really lack immediate stopping power -- especially knives of typical EDC length (say, 2-3"). Thick clothing makes slashes ineffective, and most people who get into a knife fight are stabbed multiple times while continuing to fight before they even realize that they've got several holes in them and that they're leaking everywhere.
If you're carrying a bowie knife around then, okay, we can have a different discussion entirely. But I don't think that's what most people are talking about.
Folding knives are utilitarian tools. Leave it at that, and please don't try to make them into tacticool weapons. This is also how we managed to get them banned everywhere. As much as that's unfair, well, what should we expect when you make them look like something a spec ops agent would use? Except yeah, spec ops wouldn't use folding knives, because they're not in any way suited to being used as a weapon for the reasons above.
Okay, batons? A lot better. Yes, they can also be lacking in stopping power, but you've got more reach, and more stopping power. You can break bones, and easily break an attacker's weapon hand.
These are all really limited to you being attacked by somebody with a knife or melee weapon though, which is a somewhat rare occurrence. I'd be more worried about being confronted with a fire-arm. Neither a knife nor baton will help you there.