I, as a casual player, find it depends really on if you want to get through. I find even as casual my assassin can mow through groups of enemies much faster than my sorc could. If you want to get a lot more exp quicker from killing enemies. id say assassin. that is just my experience though. also Assassins, being dps are good even for casual. My Assassin solo ran a few veteran flashpoints. i had to stealth by a few bosses since even with a healer companion it can be difficult, but i Fared much better against any enemy flashpoint or no as an Assassin. But again, that's just my experience.