Marth v MK:
MK's approaches:
Marth's options cover the approaches from MK exceptionally well; Dash Attack, Dash Grab, Dash to Shield are all covered well by Marth on reaction. Pivot DBs, pivot grabs, sh retreating fairs/nairs are all wall-esque moves MK struggles with... MK trying to approach is also giving Marth openings for dtilt, which from low to mid percents will be giving Marth 20% or so damage.
(Marth's advantage...)
Range/Priority Battle:
Whilst MK outdisjoints Marth on the ground; Marth wins slightly in the air. MK's ground beats Marth's air, whilst Marth's ground beats MK's air as well. However Marth has larger commitment in the moves he requires to counter MK's aerials.
MK won't be approaching with aerials though, he'll be using them for walling/camping/baiting purposes, or once he's got a string going.
(EvenISH, slight advantage MK)
"Even ground"
Marth vs MK on the ground is a situation of at optimal spacing, Marth has his options restricted. However he does have forward smash, a high risk move with a high pay off for any error on MK's part in timing whilst having a spacing 'fight off'. He also has a dashing transcended priority/long range move, which at high levels of play is often used for baiting (foxtrot, react, DB); This is something MK does not have. Whilst functioning similar to MK's ftilt, it has more commitment and less range, and less speed over all.
If MK is not spacing optimally, Marth can 'fight' with dtilt for creating openings, similar to mk's dtilt if it trips (but can be used at optimal spacing as its longer range). If Marth perfect shields a well spaced dtilt he'll get a free (most likely) tippered down smash. Whilst yet again a high commitment move, down smash does outdisjoint MK's dtilt, and as for it's speed [6 frames] can punish a whiff... A mix up at best.
(MK's advantage; slight or noticable)
Marth's approaches
Marth approaching MK is difficult. His approaching is 'walling' [with aerials] his way closer, finding openings for dancing blade, or dash to shield...
MK counters two of the three purely with ftilt. Walling can also be hindered by dtilting at Marth's landing, however fair can beat it, and better spacing from Marth can weave out of range inbetween fairs...
Then comes Tornado. Tornado essentially beats all of Marth's approaching options for the most part; dash to shield whilst potentially allowing dolphin slash, is risky and at great use won't be working often (this is from myself playing M2K, whos tornado usage is... unbelievable to the point if he's hitting my shield, Dolphin Slash will not come out due to the shield stun he somehow produces). Due to it's nature it's a low commitment move that is a long range spacing tool that 'counters' Marth's approaches; and to a degree his walling as well. Being pushed into a corner and then escaping is... not really fair, is all I can put it down as.
Also a quirk of "even ground", but Marth struggles with characters with great rolls; as none of his moves cover him from both sides at fast speed, any minor committment what so ever can have MK use his STUPIDLY FAST forward roll to ... make us cry, getting grabs or other moves whilst we're still lagging from the first strike of dancing blade...
(MK's advantage, oh GOD, most definitely)
Punishment game...
Marth's higher damage moves is what creeps the match up back towards Marth, slightly. Grab release for 14%ish (with many options for this, so move staling isn't to be overly considered), grab releasing to kill moves. Juggling options that MK struggles with. Dolphin Slash for bad spacing. The looming 'tipper' that can be taking stocks extremely early even if MK is power housing along. Nair and bair killing well...
Marth can cover MK's options on the edge well, but Marth cannot handle planking, because of this, Marth is most often forced to let MK back on the stage (it's not stalling keeping yourself in a safe position). However any predictions give Marths a smash attack or aerial - Fsmash for ledge drop crap and get ups, up smash/aerials for ledge jumps...
MK's grabs, nair, and gimping, juggling. MK's grabs are hard for Marth to deal with, good damage, allow FANTASTIC reading for follow ups (look at any Dojo vs Marth vid from Genesis, he literally chaingrabs us with dthrow; he did it to me as well as Bardull and Mikehaze). Nair is ****ing fantastic. Transcended priority, reasonably good range, high damage, kill potential punishment move for any bad spacing or for gimping. With Marth's mediocre recovery, and MK's many set ups or just ledge tactics, Marth gets gimped to lose stocks on a more common basis than Marth would get the 'lucky' tipper to get an early stock; from Mk's various crap: dair, shuttle loop, etc
Marth forced to the ledge is in a predicement, especially if he has to handle the triple jump glitch. MK's dash grab that cover him for a distance Marth can't "reach" from the ledge, and fsmash that Marth cannot beat with any of his moves in a trade off, plus the high chance of a gimp... MK is able to Tornado above the ledge against Marth, and it is FREE DAMAGE for MK, unless Marth attempts to dolphin slash onto the stage (hello landing lag/punishment)
Marth's weakness of below him is amplified when MK has tornado, nair and uair to set up into things. Marth with his better aerial mobility can weave away from some things, but if he's forced to move towards the stage, Marth's options are severely limited, free damage, almost.
(All in all, quirks to both sides, but it would seem obvious MK has it better)