Surely the purpose of equipping Citadel Guards with spears on horseback is to proivde the spear support for charging Knights. The two together seem to be a great combo. That first turn the cavalry hits combat will have 3 attacks with FV 4 with 4 lance attacks to wound.
After that turn the cavalry make a choice between charging with the CG to bring their extra attacks into the fight or to stay as a supporting line. This decision will be situational but by then the Gondorians should have made some kind of dent in what they're up against.
My opinion is that a mix of the two will be far supeiror to one or the other.
In regard to equipping the guards with longbows, they're a great weapon and add some decent ranged ability to the army. How the horse archers are used will vary, some armies they'll be great for scoring some early wounds against others it would be more advisable to forget the bows and engage quickly.
If you've got both ready Steve why not try and practice with them on Fri (assuming your free) and make the decision that night