What phone(s) should I buy? Or not?

I've been debating my phone strategy: I now have a two-generations-old iPhone on AT&T and a Nexus One on T-Mobile plus an AT&T laptop card.

Do I buy the new iPhone 4? Do I buy a new Android? Do I shift on Android from T-Mobile to Verizon? Do I move to one phone and platform? If so, which one? Do I get mifi? How do I convince my wife it's not insane to buy new phones? (The last one's the toughest because when she looks at me like I'm crazy, she's right.) So here's my rationale and...