I have a 2010 Macbook Pro 15″ (I found the 13″ too small for something I was going to use alot). Still going strong, runs Mountain Lion fairly well. I use it every day for web design. You can pick this vintage up _fairly_ cheaply – I’ve seen some for around £500. However, if you can stretch to a retina model that will be more future proof.
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I’m with the Mac Book Pro crowd. Sticking a $90 SSD into a MBP gives you a significantly beefier system than the Air for a lot less money. Having used both a 15″ and 13″, neither of them are really optimal in terms of screen size, so I’d just go ahead and save the money for the smaller model for more portability when you need it, then grab an external monitor. So my vote is a the refurbished 13″ for $929.
What @joshuanhibbert said – this is what I had done instead of buying the iMac. I should have gone for a hardcore Macbook Pro and bought a sweet monitor, still would have been around the same price but portable too.