Had a 15" for 2 years, went to 17" and am happy. Not that much bigger/heavier than the 15", but a lot more screen real estate. It also lets the same processors run faster than they do in the 15" because the 17" has better thermal characteristics.
People thinking about the 15" should spend time with the 17" to get a feel for the compromises they'll be making if they get the 15".
Now that I have a workstation-class laptop, I wouldn't mind also getting an MBA for just iOS development -- the current MBAs are too underpowered for my full workload.