Marth has the advantage. I play both and I would never pick a Kirby against a Marth. Kirby has some nice air game but on the ground, unless he can get in close, he's screwed. Marth is a bit faster and can keep Kirby at bay. He's a beast at edge guarding plus, depending, he can have priority and knock you right off the ledge.
The one nice thing about Kirby and Marth on Kirby's side is the fact that we have so many jumps. Since Marth up b doesn't have a very high range, it's very easy to get a Kirbycide on him and take him down to where he can't come back. It works really well with a spot dodge or roll behind (depending on the move) and suck from behind. Then you can shoot him under the stage and his chance of recovery are very slim. But at the same time if this isn't timed right he's got the ledge again and can spike you.
Keep Marth off the ground. His main game is obviously his ground game. If you keep him in the air you can generally wrack up a lot of damage. Plus, if you get him stunned you can get your grabs in. Do not do a down b on him. Unless you have him completely incapacitated, his up b can get you before you completely stone and he's faster than you when you come out of it so it can be punished really easily by Marth. You can get him on stone slides though so if you have the opportunity to do that then try, but make sure you get off the edge before coming back out of stone, especially at a high percentage because a smash will kill you. Plus you'll have all five of your jumps and if he is edge guarding, do an up b, just make sure you don't hold down (as we all found out.
) so you grab the ledge and make sure you do it low enough so he can't punish mid-frame. Although your safest bet would probably be to just jump up and hammer him away from the ledge. He can match it, but it saves you and allows you to get back on the stage and shield or dodge the next attack.
If I think of anything else I'll write it down but I'm a bit slow this morning with the thinking. >_<
OH! Stages: Ban Battlefield. Ban anything flat. Smashville is good because of the stone slides... Don't play on Corneria. Um... Japes wouldn't be bad against him, but it isn't the best either. Pretty much just nothing flat. OH! And no PS. There are too many options for you to get completely screwed on that stage.