Just his acting in Gone Baby Gone and The Assassination of Jesse James (especially the latter) destroy anything that Ben has done (with the exception of Good Will Hunting, and that movie was good in spite of Ben's acting... great writing though). Ben should stick mostly to directing and do crap like "He's Not That Into You" to pay the bills if he needs to, Casey's got the acting talent in the family.
shareAgree with Sarajw13,
Ben Affleck made some wrong movie choices - he's at his best in slightly goofy/funny roles. When he plays the all American square jawed hero, he comes across really wooden.
I think he also committed career suicide when he was dating Jennifer Lopez, and the whole 'Bennifer' things started. Then "Jersey Girl" went t*ts up and got panned, when it actually wasn't such a bad film but by then the J-Lo thing had gone stratospheric.
But Casey's definitely got something special, he's way more interesting than his brother. Maybe Ben should stick to directing Casey....