It doesn’t matter which one you use. The important thing to remember is to use a string as close to the missing one as possible. I’ve seen harps where the harpist was missing a 1st octave A for example, and they replaced it with 2nd octave A, thinking it was better to stick to an ‘A’ string.
The tension will be marginally lighter if you use the next string up as a replacement, and marginally higher if you use the next string down. But I doubt in either case that you can feel it.