I find that firearms are better for general use, and most cannon-type weapons are devastating in sieges. (Though I'm still not entirely clear on how Rockets work.)
I usually arm companions with a light arquebus to start with if they aren't already suited for another weapon type like Deshavi or Klethi. They're more likely to kill an enemy when the first time they hit them and get them the experience they need. I upgrade the weapon later if I can, of course, but light arquebus' are readily available as loot from bandits in the "Pack of Villains" groups you see earlier on. As for armor, I usually farm the Taiga Hordes for their Ruski gear, then just buy them better stuff when money isn't an issue.
Blunderbuss', grenade launchers, dragons (the pistol type) and the like are pretty much my go-to siege weapon even when I'm playing as the mercenary archer. Being able to regularly take out half a dozen enemies (possibly many more) with each shot is pretty incredible. When you don't get blasted by an Aleph the moment you start, anyway.