FfH:
If a unit in the stack has Guardsman, any attacker with Marksman essentially ignores the fact that he has Marksman.
-----BUT!-----
Any unit with Guardsman is more likely than normal to defend the stack against ANY attacker.
Pro: Pretty easy to figure out who will defend, because it will essentially always be the Guardsman.
Con: If you get Guardsman on a REALLY weak unit, and defend with REALLY beefy units, then nobody can hit the Guardsman. Also, if you do not defend yourself like that, your opponent can attack with non-Marksmen units, take out the Guardsman, then come in with his Marksman and wipe up your mages.
-----------------------------------------------------------
FF:
If an attacker has Marksmen, he will ONLY see units in the stack who have Guardsman, and pick the best defender from that group alone. So if you have a single Guardsman at 0.1

in a stack of 50 units with 20

, Marksman picks off the Guardsman. But if you have multiple Guardsmen, they work to guard each other.
If of course the stack doesn't have Guardsmen, the Marksman attacks the least likely defender as usual. And if the attacker doesn't have Marksman, the Guardsmen defend against normal units normally, the Guardsmen promotion doesn't effect anything (well, unless they are in a city of course, where it enhances the defense for everyone)
Pro: Guardsmen aren't sitting ducks against rank & File, and won't be COMPLETELY suicidal if you have multiple Guardsmen. Also, the Marksmen MUST deal with the Guards themselves, or you have to completely dominate the stack (or be lucky enough to have the Guards be the strongest in the stack, letting rank & file troops attack them)
Con: It might be somewhat tricky to figure out precisely who will defend for you against your opponent (I personally don't think so though, but this is as close as I could think for a con), but you do always know that if they use Marksmen, you'll defend with a Guardsman, and if they don't, you'll defend normally.