Singed seems to be more of a high-class villain crazy - he's chill, smooth and cool, and then, when you least expect it, he's got a diabolical plan. He's mad enough to inject volatile chemicals into himself, but not stupid enough to overdo it like Mundo. He's insane, but smart.
Mundo, on the other hand, just wants to hit things 'til they die. He doesn't care for anyone - not even for himself. He can be described as bat**** ****ing crazy; he just wants to **** **** up and see what happens. He's mad, he's bad, and he's dumb as hell, but at the same time, he has an animalistic cunning.
So, in my opinion, Mundo is crazier, but Singed is more dangerous. They're both nuts, but Singed does the maniac dance with style, while Mundo just drools.