I prefer to have a Spearman in the stack with Combat I/Medic I.
The Spear can ably defend the stack from mounted units, first off, so I recommend including a Spearman in a stack regardless of how you choose to promote him. But in addition, unless the attacker is a mounted unit, the Spear will almost always be the LAST unit chosen to defend the stack from attack because he's weaker (strength 4 rather than 5, and enemy Axes get a bonus against melee units like Spearmen). If you have a stack of Combat I or City Raider I Axes and one Combat I/Medic I Axe, the latter will get chosen to defend just because he has more promotions. Thus, your medic unit is more likely to survive to heal the other units if it's NOT an Axeman.
And as Betafor pointed out, you don't really need a Medic unit that moves faster than your slowest-moving attacker. I prefer to use fast-moving units for scouting and pillaging, and they benefit from different promotions (Flanking, Sentry, Combat) for those purposes.
As for Medic II versus March, I almost never split up my stacks, so Medic II is of limited value to me. Dispersed stacks just seems to invite suicide Catapult attacks that weaken all of my units rather than just a few of them. There also isn't usually enough good defensive terrain to go around. I'd rather ensconce my stack on one forested hill than have several units inviting attack out on open terrain.
If I'm facing a lot of mounted units, I'll often forgo March and give the Spearmen (and, later, Pikemen) Combat and eventually Formation promotions.