generally i go for a lightner s/i because it solves two things for me
where tenna winds up in the light world where he is guaranteed a sense of stability (which i don't think he gets with mettaton tbh)
why he's obsessed with my s/i. i think for a darkner partner to really stick with tenna they have to absurdly match his freak. rose does in ways but for some parts of her personality to work lightner is more logical. the caveat is that i actually find the lightner daily loop where they just "hang out" potentially boring so i still have to think of a plot/conflict i personally want to do. i have a year's worth of ideas just nothing solid
but lightner brings its own set of issues. i'm not convinced a fountain could be made on accident
sometimes when i'm daydreaming i sacrifice logic for self indulgence