DK wins this 55:45. Trust me.
Once Marth is in close, DK still has jab, dtilt, and downB, which can be beaten, but doesn't mean they will by default. Let's not forget DK can gimp Marth too. His recovery is very good, and isn't as punishable as we'd like on most CP stages. DK dramatically outlives Marth, and a GOOD DK will NOT DIE unless Marth tippers him, the end. DK can kill Marth relatively easily, and if he calls a Fair at any given moment, can kill with Giant Punch. DK has small combos, but they rack up a third or a quarter of the percent that he needs to kill Marth. DK can also trap with Fsmash, which is counterable, but it's also risky to counter, because if he charges it, you're eating SO much damage. DK's UpB is also strong on it's own right, and racks up crazy damage, or can gimp if you mess up an edgeguard.
Marth can combo DK, and even 0-death DK (difficult, but possible). He pressures and punishes very hard, but has trouble landing the kill. Marth has counter and Dolphin Slash, which save him from high pressure situations. He also has shield breaker, which is very useful for it's range, power, and it's ease in landing on DK. SHNairs pulled off the shield will Shield stab the second time, and assist Marth in landing a K.O..
All in all, the scales are slightly tipped in DK's favor. Thank god he gets comboed.