I dont like using ratios, but I think this is 55:45-60:40 Marth's advantage. I play this matchup A LOT, so here goes....
Ground games:
Falco got the camping advantage, but forget up close. Marth pretty much wrecks falco unless Falco is within grab range...
Interesting note though: Your Fsmash has as much range as Marth's, if not more.
If marth gets close, get away. Your best spacing tool is ftilt. Don't be aggressive for the kill especially at high percents. Marth has one of, if not the best, up-bs OoS in the game.
Aerial game: Marth's main weakness is below him. He has a harder time than most getting out of juggling. Of all your aerials, upair and bair are the best for aerial attacks imo. Nair and dair should only be used in shs. Ex. Nair through platform, dair after cg, etc. Your bair can't beat his fair, or nair, but it still has plenty of range.
Grab game:
Ok, both sides have low percent chaingrabs, but most already know this.
Falco: 3 buffered dthrows from 0% to dair
Marth: 0-death near the ledge or 0-27% on stage(2 throws to tipper fsmash).
Its not worth going for the grab; please dont. Camp the center of the stage so you don't get 0-deathed.
Mindgames: You could always do one or two buffered cgs, and stop. If the marth was mashing upb, he will do it while your shielding, and free fsmash or whatever you want to do :D
Offstage. This is where I hate marth. If the marth knows what hes doing its a pain in the *** to recover. He can come out to fair and nair you, or stay on stage and try to bait an fsmash. One tactic the marth I face all the time uses is this: Stands right next to the ledge so if I go for it he ledge hogs. If I go on stage I illusion right into a dancing blades combo or an fsmash/upsmash. If I go high he upairs. As Larry said, you can reflector him to edge guard, try a bair(risky though), or just spam lasers.
Stages:
Dont take him to stage with a lot of platforms(like bf). Keep in mind a platform is nice to have so you can camp it to avoid 0-deaths. Cps would be FD, SV, Ps1, and maybe even yoshi's if your willing to put up with the disadvantages you receive too.
If you want to take him to a stage like ps1 or delfino, be wary of his dtilt lock on walls.
I think halberd wouldn't be bad either if your willing to put up with the advantage he has on the first phase.