I think it's more a matter of personal preference for me: lately I've grown tired of riding to battle and prefer infantry combats and therefore prefer Vaegir Guards. Next week...who knows?
Your list is kind of short but interesting. My opinions, which are influenced not so much by weapon choices or armor or "stats":
Sword Sisters are really really cool. I usually have them in my party. Biggest drawback is that that you can't recruit replacements, you've got to win their freedom. Love the white coursers.
Hired Blades are really my choice when I'm a "neutral" character since I can recruit and train them straight outa Zendar. "Have Blade. Will Travel." Their Stats are fairly good too, for that matter.
Swadian Knights: never cared for Swadian units, not now anyway! Maybe 2 weeks from now, I'll go back to having Swadian sharpshooters and Knights again. The Swadian Knight stats are fairly decent also.
IF I had to choose for efficiency or power, I'd probably give the Swadian Knights the edge because they're easier to recruit (more places to hire from), have decent units all along the upgrade path (the Swords Sisters for example are fairly lightly-armoured through most of their upgrades), and seem to be the most heavily armored and well-mounted at the end (Sword Sisters are often helmetless and Hired Blades sometimes show up on foot).
Oddly enough though, I don't use them. I like variety over efficiency. Just my opinion.