few people are answering because its a question that has been asked over and over again, and its mostly a matter of oppinion.
my oppinion
you companions aren't smart enough to bob and weave or use any other tactics, so when fighting against opponents with missile weapons, shields are a must. A second for their back is a good idea if you can manage it, but if they can use a missile weapon, you might as well let them, which means some will have to forgo the extra shield for a bow and arrow.
the key to leveling them is getting them to kill, since they are initially pathetic, speed seems more important than damage in a weapon. they will hit more, thus swinging even faster, and it will increase the chance of them hitting before their attacker does. i usually go for a balanced scimitar, although i sometimes use the balanced military pick whem i find myself fighting more heavily armoured opponents.
once you get them up in level, say 15-18, and you find yourself fighting more sieges, you might consider switching to a two handed long reaching damage dealer, but be prepaired to watch them fall against missile heavy troops like kerghits and vaegirs(just like anyone else without a shield).if you save some of their weapon skill points it will make the transition easier(besides after a while the cost isn't worth it anyway)
get them the best armour you can for survivabilty, i usually go winged great helm, heraldic mail with tabbard, gauntlets, and iron greaves. it costs a fortune though. And remember, regular troops are either wounded or not, your companions have a damage percentage like you, best to keep them out of a fight when they fall too low, since they will just get knocked out before they can do anything anyway.
they can be very effective in combat, they can out perform top teir troops, but you have to invest properly, outfit expensively, and baby them a little more than you do your nameless grunts. but its definitely worth it, if only for the secondary benefits.