Silverstag currently uses the Native troop tree. It is currently a bad idea to stack anything but heavy cavalry or heavy infantry. The improved AI means that in lord battle, infantry can make shieldwall, so archers are only viable when in small scale battles. They run out of arrows too quickly in big battles while doing very little damage. Heavy cavalry also makes the party more mobile on the overland map.
Khergits
All of their units are cavalry except at the lowest level. However, their top tier melee cavalry is weaker than other heavy cavalry.
Nords
They only have infantry and archers. The huscarl is by far the strongest and fastest infantry unit in the game.
Swadians
They have archers, infantry, and cavalry. Their cavalry is the toughest to kill.
Vaegirs
A well balanced troop tree. They are a lot like the Swadians but with slightly stronger archers and weaker cavalry.
Rhodoks
They have infantry and crossbowmen exclusively.
Sarranid Sultanate
The majority of their low level units can throw javelins.