Personally, I go back and forth between point buy and rolling, but I'm interested in the question between monk and sorcerer.
Let's say we're making a sorcerer and we end up with, oh, 18 12 12 10 10 8. He's going to be pretty powerful with that 18 - nice work.
On the other hand, we could make the Monk with the same numbers and he'd be pretty good too. Throw the 18 into Dex, and take Weapon Finesse, and he's hitting a lot. Put the 12's in Con and Str for a little damage, or Con and Wis for a little extra AC. Alternately, throw the 18 into Strength for the damage and to-hit boost, and concentrate the 12's into Dex and Wis (or Con and Dex) for AC, and keep your eyes out for something AC-boosting (like that sorcerer) that can help you out.
I think a Monk with the same attribute numbers as a sorcerer can do very well - although with point buy he may do a little better by going from 18-12-12 to 16-14-14. (I don't have the exact table here.)