What would be best in diablo 3: level up one hero at a time or keep all heros around the same level?

If you play by yourself, I would suggest playing around on different toons.This will hlep keep you from getting bored of playing just the 1 class.

If you play with friends, I would try to keep at least 1 toon at the same level as theirs (usually means having a good one at least to Inferno).