Tbh after seeing a few games, analyzing what Marth can do, and reading (and getting salty) on the Marth's weakness's topic on this forum..
I can say, with a (slightly) informed opinion, that Sonic is -2 for poor Marth.
Let me explain my horrible reasoning.
Marth, while he DOES have an incredible defense game, really safe moves, wonderful mobility, and a decent recovery. Sonic, hes a much safer neutral, much faster frame data, and his mobility makes it really difficult for Marth to catch Sonic with tippers, and even harder to kill the little blue screw ball unless you're just blatantly throwing out fsmash and usmash like its your birthright, Sonic can force Marth to play on the offense, if he plays defensively, Sonic is going to win that battle of defenses. Sonic can also drag Marth to his knees with Spin dash, yes, Marth does have the tools necessary to combat it, the problem is actually timing them so it hits, that is surprisingly hard for Marth, as nair goes too high, fair also goes too high, the hitbox JUST BARELY misses. Marth has to resort to Jab and his tilts, and to get the tipper on any of them while Sonic is spindashing, you pretty much need the reactionary skills of a house fly. If Marth can get a decent lead on Sonic, he probably won't catch up, the problem is actually being patient enough to catch him and put him down without getting screwed by Sonic's really (??? idk) good throws. Sonic can also punish whiffed moves (not things like his tilts, but things like his smashes, dancing blade, and his aerials if fastfell) from over half stage away just due to his absurd running speed. Another thing to point, Marth has trouble landing, but with Sonic, it's amplified, The hitbox on uair is really big and long ranged, and its not like Mario's uair, you can't just cut through it with dair, and if Sonic is put into a bad position aka being juggled, well one spring to dair ought to end that right quick. Now, about the killing competition, Sonic is able to get damage on Marth much faster than vice versa, but killing is a whole other story, Any of sonic's finishers are easily walled out except bair, which allows Marth to catch up and Maybe get an early kill if he DOES get a tipper. Marth better not let his guard down though, Sonic has 3 kill throws, bthrow in particular being great, especially with rage. Sonic also can get in on him for near free as Marth lacks a projectile and a fast get off me option, Sonic can also retreat nearly for free if things get ugly for him, campy sonic is he(ck) for Marth to deal with, as well. As Sonic can run away from any of Marth's attempts to space, and run right out of range but still in range to get the jump on him if the Marth gets impatient.
Now for the good things for Marth in the matchup, Marth has a really big range advantage, not as big as Mario though, as Sonic actually has better range than Mario on almost all of his moves. Back to the point though, Marth can space rather well against Sonic in the air...Most of the time, Sonic can beat Marth's range if he gets up in his face to use his aerials, another thing to point is, Sonic has to be weary of using Spindash, yes I said Marth has trouble punishing it, however, if Sonic uses it randomly, Marth can dancing blade it rather easily. Sonic also has to be weary of Marth's tipper, as one wrong move, smash, etc, and it can lead to an early, VERY early, death. Marth's jab is also a very viable tool in keeping Sonic out, it also leads into dancing blade which can put heavy pain on Sonic. I don't have much for killing without tippers, however. As Sonic will usually kill Marth beforehand, Sonic's bthrow DOES kill earlier than Marth's uthrow.
So there you go, horrible opinion.