I just found an open source window manager called slate.. yes its better than divvy and better than sizeup.. and it has a key binding for switching focus between screens. It's default key bindings are shown here (very customizable).. it defaults to ⌘→ and ⌘←
note: Don't listen to those who way slate is all complex blah blah.. it's every bit as simple as the two other solutions.. it's just those two are a bit more visual. that's all.