From my own gameplay it seems the Swedes have the best equiptment but are expensive and have some of the lowest skill points, have good two-handed swordsmen/pikemen. The Cossacks have the highest skill levels for their riflemen and have decent firearms (not much armour though). The Polish are all about their Hussars which can flatten armies, Dragoons/Zoniers are alright as well. The Russians are more of a well rounded faction which are also fairly cheap, decent Reiters. The Tartars are focused on archery, mainly horse archers and foot archers which can be interesting, have some decent rare riflemen but go down easy to having almost no armour.
I had the most fun in WFaS playing the Cossack storyline as a sterotypical Cossack Rifleman. Having an army consisting of mainly riflemen and having them stand in ranks and shooting in volley just at the prefect time to stop a charge is pretty fun (would be nice to have one of those "improved moral" systems from some of the more popular Warband mods though). Have to watch out for the Poles with that tactic though since they have the tendecy to just ride straight over your linesmen, having some pikemen can help but once those Hussars charge it can be a bitch to stop them.
Playing as a Tartar horse archer is also very fun, shame the Swedes and Tartars lack a storyline quest.
As for the upgraded mercs from camps I'm unsure if they're stronger than unique faction units. It's true you can really outfit the mercs with some great gear but they have fairly low skill point levels and can be very expensive to hire/upgrade. It can be rather hard to get the faction units though so the main part of your army will be mercs and some village cannon fodder. Easiest to just recruit as much prisoners from bandits/lords as possible since you'll be losing a lot of men from rifle fire/fucking Hussars.