Or do you think that 3/4 and 6/8 each have a unique enough feel that a 3/4 song would still feel like 3/4 even if you bump the tempo way up, and a 6/8 song would still feel like 6/8 even if you drop the tempo way down?
people may even disagree about what to put on the page. i see "3/4" written all the time when the quarter note is at many hundreds of bpms. on the other hand, often i'd see 4/4 in the same situation, using triplets and then the quarter note bpm would be extremely slow. for that, i usually prefer to compromise and write it in 6, to adhere to a good size bar length. i generally try to keep the bars as close to 3 seconds as i can, to avoid feeling disoriented