Some people say that the NPCs are hopeless in melee combat - especially early in the game, although aren't we all? - and they prefer to give Borcha and Marnid bows/crossbows to keep them from soloing into a swarm of enemies.
They do tend (in the beginning) to get knocked out fast; but with better armor and weaponry they can last enough to make a contribution.
I have no strong preferences on how to equip either; of course if you're at the stage in the game where you have access to black armor, spirited warhorses, swords of war, sniper crossbows - then yeah, why not give them the best? Who wouldn't be more effective? But ....sigh... that's not an answer 'cause what do you do the other 90% of the time?
Generally, I make sure their gear is near equivalent to mine. I prefer they be mounted using 1H swords and shields, as the dynamic duo tend to get involved in upclose melee combats where weapon & shield are somewhat more useful. If they had the brains to charge in and out of combat I'd give them lances or spears.
I've seen them use most weapons successfully at some time or the other so it's not particularly crucial what they use.
WHAT IS MORE IMPORTANT than what these NPCs carry, in my opinion, is what PARTY SKILLS they can develop: tracking; pathfinding, healing. I think their contributions are greater in that way than in their combat abilities. Here's a good thread: http://taleworlds.com/v-web/bulletin/bb/viewtopic.php?t=1113&start=0&postdays=0&postorder=asc&highlight=borcha+marnid+useless