I spent 200 xp to complete Squire, so I have 200 xp left. I could spend 100 xp on AK (Genealogy/Heraldry) as well, since it's in character and I'll need to learn it when he becomes a Knight anyway.

So I have 100-200 xp left to spend, which will allow me to enter either career immediately (200 xp for Apprentice Wizard, 100 xp for Knight). In terms of RP, Knight is probably the most obvious next step, since Cassian is literally training to become one, but Apprentice Wizard is necessary for my ultimate goal of building a fighter/magic-user. It also makes him a more rounded character in the near term - he gets to boost Int, WP and learn to read/write - and is probably cheaper to complete than Knight.

All in all, Apprentice Wizard is probably more useful in the short term, since he's already a pretty good warrior (unless you're about to spring lots of combat specialists on us )