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I don't use portable mode for the Switch often, but I'll be going for that version of the game. Sure, the framerate is lower than the other versions (30 instead of 60 fps), but it's good enough to me imo. I was used to the former framerate in Bloodborne.

I suck at Dark Souls, arf. I got it on PC and only managed to get that goat boss, arf. I got frustrated to walk all the way to him in order for me to get kill with 2- 3 hits and start all over again, arf. I really love the dark fantasy setting, but the difficulty and backtracking is way too tedious for me, arf. I would only get the game if they offer some kind of easier mode, otherwise I will keep on skipping the entire franchise, arf.

And to answer OP's question, arf. I have no idea what the visual difference will be, arf. So far, I would go for the portability aspect, arf.

I've played OOT with a higher frame rate and it didn't play better. It played "differently" but not better. Do you know why that is? Because "better" is subjective in the way you're using it.

My priority will probably be whatever friends get it for, so I can fight bosses/invade other peoples' games with them.

If that wasn't the case, however, framerate would probably be my priority. While priority is generally great for gaming, Dark Souls requires a lot of focus, and when I'm taking a game on the go I'm usually not dedicating my full focus on it at all times. 60 FPS, meanwhile, is absolutely beautiful; I go out of my way to play FE Warriors docked because of how much better it is than the 30 on the go.

60fps is better it's not subjective, more responsive controls, and smoother.

...Just the opposite, dude. I'm 99% sure he's using 'play better' in the measurable, objective, performance context. A more responsive game plays better much like a higher fidelity video 'watches' better.

He's not. He just said (above you) that because a game is smoother, it's better. No, it's not better... it's smoother. There's a difference. Just like your example of higher fidelity video "watches" better. No, it doesn't. It "watches" more CLEARLY though... which you may subjectively enjoy watching more. 

The point is, frame rates improve motion which you may find makes a game play better but other people may find the opposite true. For instance, I think OOT plays worse with a higher frame rate because it changes the "feel" and movement of the game. I think the original game played better. That's my subjective opinion.

The PS4 in portable mode plays at 0 fps :p

In reality (assuming you don't mind playing undocked) which version would you really play more in real life situations? 

60fps also means less input lag, more responsive controls. i honestly don't see how anyone can prefer 30fps over 60fps, its just a huge downgrade and feels like the game is chugging  30fps is playable, but playing the same game at 60fps, then changing to 30fps, you can feel the huge downgrade right away.