So far as I can tell, hiring the highest level possible adventurer is always best. Since experience is shared, your lower-level party members will always be behind, and if you get two companions at lvl 2-3, they'll be at most 3-5k XP behind, which is a miniscule difference at level 5-7 and beyond. If you can tank encounters as a Paladin or Fighter, it's easier, but even with a ranged/low Health & Endurance character, it's better to get friends early rather than late, as the differences in XP grow as you progress. As a general rule, since any hired companion will be a full level below your last full level, it's best to hire any companions right after you level up.
