For clarification, I guess what I am saying isn't that ethical merit translates to political or military success, but rather that not
being evil in dumb ways does. That is another thing entirely, of course.
Yes, I believe that the Tywins in real history may very well often be successful, but the Joffreys are not, really.
And George R.R. Martin doesn't really portray Joffrey as successful on his own merits, really. The other Lannisters weren't impressed at all by him having Ned decapitated, because he wasted a very valuable bargaining chip. One doesn't have to be as stupid about being cruel as that for it to come back, though. I don't think so, anyway.
Tywin calculates the politcal and military pros and cons of doing cruel things before doing them (his treatment of Tyrion is the exception to his nature of being calculated about cruelty, and well...). That is why he is largely successful. So far, anyway.
Joffrey just does cruel things because it is in his nature. He is a teenage bully and prick who has cruel whims. He wouldn't be very successful at all if other Lannisters didn't play the game of thrones for him.
Those are two very different philosophies about being cruel to others.
Tywin even lectures Joffrey about this a couple of times, and it is not about making Joffrey a more ethical person, but rather when and to whom one should not be cruel.
But yes, in terms of military success, the pros of doing the Red Wedding almost definitely outweighed the cons, for the Lannisters.
In short, when comparing the Neds, Joffreys and Tywins of real history, I should think the Tywins would be the most successful in power struggles in general.