If you're like me and walk into a clients business and show them some final touches on a website design via localhost, get an air.
Granted the Retina Display on a pro but that's about it, for the nimble mobility of an air it's hard to beat. Talked my friend into taking his pro back for the air
For desktop I got a dual display 27" iMac setup.
I'm typing on a dual monitor windows machine right now.
Even though os x has parallels I still like having one OS per box as there are less things to worry about, especially with OS updates.
