Very even matchup but if anyone has the edge it's Marth. Sure Snake does big damage, KOes earlier and has the advantage in neutral but Marth is really deadly once he gains momentum (Snake gets comboed and juggled really easily by Marth) and CAN gimp Snake sometimes when he gets Snake offstage at not too high % (at high percents Fair hits Snake too far). Marth also has much safer tools than Snake does. Snake's Ftilt has a lot of range but isn't really a safe poke tool even at max range vs Marth because it's unsafe on shield. Snake actually can't punish a Marth shielding that well in mid range except by commiting with his risky, mediocre dash grab which loses to just about everything else Marth chooses to do in that position. Other than that Snake can just throw Grenades, wait until Marth commits with something (which you may have to read which kind of commitment he chooses) or wait until Marth's shield gets smaller.
