You have three options, Air, 13" Pro or 15" Pro. The Air is a 'cheap' Mac for people who don't do serious work, its only dual core so discount that immediately.
What's your budget?
Are you looking new or used?
Will this be a primary machine?
Does it need to be a laptop? If not a Mac Mini may be a more economical fit. You could get a Mac Mini and a decent Windows laptop for the same price as a 15" Pro (new).
I suppose it depends if you are looking to use Apple's development tools exclusively. 3rd party tools tend to be cross platform which is where the mac mini comes in again.