Survivor tends to have half of its cast over 40. It's got its share of eye candy, sure. But Big Brother has never had more than one person over 40 in the same season. Also Survivor just looks for assertive, interesting personalities. Big Brother specifically looks for spoiled rich entitled drama queens.
And also the editing. Take Hayden. On Survivor he seems like a perfectly normal, decent guy. On Big Brother he was like MICHELLE IS EVIL AND CAN DIE WE ARE THE BRIGADE EVERYBODY IS EVIL FOR NOT DOING WHAT I WANT THEM TO DO! I'm pretty sure they make them trash each other in the DR.
And yeah, they are fundamentally the same game. Vote people out one by one. Though the HoH dynamic is a big difference, it makes it harder to play a dominating game and easier to get through on a quiet social game. But in terms of casting and editing, Survivor wins by a mile. And Survivor doesn't do stupid crap like "The Ex Factor" as often. At least, not until this season. Survivor's 'twists' tend to be changes in gameplay mechanics like tribe swaps and hidden immunity idols, not stupid forced relationship twists. (Again, the current season is a bad example)
I suppose they did do that stupid 'Race' season.