I resisted temptation to get a new laptop and just got a 500gb solid state drive for about £270 (approx $350) - boots up in secs and new applications open and close real quick. The laptop looks a bit shit though.
I fell into the same mistake. You know SSDs shrink over time, right?
I would get a highly customized dell. The best way to get a discount is to simply start up a chat on the dell website, and tell them exactly what you need. I happened to get $550 off my order when the person said I'd be getting a processor that doesn't exist. I even gave them a link to the Centrium website to prove it lol. But yeah, go with a dell.
My iMac is great for screen real estate and everything, but I don't get the programs that I want (WinSCP and Notepad++). I can't really live without those. Also, I don't know how someone could develop on a Macbook Air. Those are better used for travel / conferences -- not exactly a performance machine.
And the fans on Macbooks are under the keyboard, so your computer goes to sleep whenever you shut the screen/lid. Terrible design!