Possibly 6/8 was more favoured by past composers, as Jack has said. Scott Skinner's "Hector the Hero" is written by himself in 6/8 (original score on Aberdeen University website), but most modern scores are in 3/4.
Perhaps the more important thing is not to classify all 6/8s as jigs (and here in Scotland we have a lot of dotted 6/8 marches). And not all 3/4s are waltzes: many are slow airs.
In the end, it's more about how you play it.