i think that if the spacing and range between the two is that close, then perhaps this should not turn into a battle about using marth's range, and from what i understand, MK is basically the Baron of Brawl, having almost total priority in the air, therefore, i would conclude that playing an aerial battle, despite marth having great aerials, isn't the best idea...
the other thing is that MK is useless after every B move he does: the 'nado, the shuttle loop, etc., making these opportunities, if marth can keep away from them, the best moments to camp the crap outta MK...from every MK i've played, the 'nado has been a staple of their game play, but i find that this isn't a very big deal, i've simply either kept my distance, or (dangerously enough) ran into it and countered it, leaving MK open for a few frames for a marth to try and sneak in an aerial, considering how useful is Fair is...the other thing is that his shuttle loop you can see coming, once that up B is thrown, its clearly visible; however, if the ending attack will be utilized is a different story
the other thing is that even if marth has range in some cases over MK, from what i understand, MK can throw almost all of his moves in 2 frames, whereas marth puts out his quickest in 3-4 frames, so outranging Meta Ghey (lolz) is still not an option...more experience from my game play is that i tend to be aggressive...while marth isn't always a heavy hitter or a killer (like G-dorf or Ike, for i.e.), if i hit someone off the stage, i will chase them off the end and have taken stock by simply jumping out and fair'ing them away and being able to recover w/o a problem and i have used this tactic to keep meta knight's oh so OP'ed recovery to a minimum
if anyone has any comments about this, please let me know (in a kind way, please? ^.^) i am new to smashboards, but far from new at brawl/melee and i would love feedback as a fellow marth mainer