1) Magnus (or any other follower with fewer levels than you when they join) will always remain that many levels behind you... until you max out at level 50.* Then, every time you would've leveled up, your followers will each gain a level until they reach level 50 themselves.

*This does not apply if you've chosen to install the level cap remover, which lets your PC continue to level up.

2) Magnus's story is better than Chukka's. (Generally, any NPC with an actual speaking role is worth considering over those who only talk via text.) He will interact with several other NPCs (including Virgil and Loghaire) and is actually at the center of a fun, but rather involved, side quest you can choose to do.

3) Magnus's tech skills allow him to craft items for you to use or sell, even if you're a mage. You just have to hand him the components and say, "Pretty please."

4) Chukka is a goody two shoes. Cross that line once too often, and he drops you like a hot rock and hightails it back to Bates. Magnus is a bit more forgiving (in all matters other than P. Schuyler & Sons) and can often be persuaded to look the other way with a bit of a bribe of Dwarven snuff.