There are several different kinds of mercenaries.
Vaegirs enjoy the best ranged units in the game, and their bows can be of use in a wide variety of situations. Swadians specialize in heavy cavalry units, giving them the most punch in straight-up cavalry charges, especially against infantry. Sarranids are the most well-rounded of the main factions, having no specific area they excel in beyond the other countries but no glaring weaknesses aside from rather light armor. Rhodoks specialize in foot soldiers using polearms, giving them the greatest natural defense against cavalry. Nords specialize in close combat foot soldiers, making them especially deadly in siege combat. Khergits specialize in light cavalry units, making them great scouts and flank units. The troops trees for your selected set may be found in-game under: Reports -> Reference Material
Different factions' troop trees specialize in different areas of combat, while others are balanced. Each of the six main factions have their own unique troop tree, and there are other troop trees besides. Troop trees show the progression of units for a given faction or group. 12 Bandits (and Manhunters) Troop Trees.