So yeah, I recently made a Magnus. So far, it has been absolutely awesome to have in tier 1 & 2. Especially against Devilites, the enemies I have the most trouble with.
The best thing about the Magnus, I suppose, is it's ability to put Fiends and Beasts in a state of perma-knockdown lol, plus the added stun chance.
So, would anyone recommend it? And if you do, should I go with the Callahan or the Iron Slug?
This is a frequently asked question. Try a Google search such as callahan iron slug site:forums.spiralknights.com. You will find many threads.
Iron Slug is regarded as one of the worst guns in the game (I don't have it). Callahan is pretty weak, especially since you move soooo sloooowly while charging. But Callahan is fun for sniping silkwings. The bullets are very fast.
Also, the supposedly-impending gunner update is supposed to alter the balance of these guns pretty dramatically. See the Testing Feedback forum.
I recommend that you eventually get a Callahan, once you have a good selection of the more powerful/useful weapons in the game.