New MBA or MBP?

If you're on the move frequently and will benefit from an ultra-portable device then go with the Air. Obviously the MBP is a high quality, proven laptop but the tide is turning and SSD is the wave of the future. No moving parts, no noise, minimal heat and with 256Gb that's enough for most people's needs. You want it, go for it...get the Air !!!

You can debate power and SSD advantage but since you're a developer, I believe the main question should be "Can I do with only 4GB of memory ?".
If you need many applications open at the same time including an instance of MySQL, it may not be enough for you. an MBP can be upgraded to 8GB of ram. an MBA stops at 4GB.

The second question should be the screen. again - if you have many windows open, the higher resolution of the 15" 1680x1050 will better serve you, especially if you have side by side windows open.

and then there's the power and speed issue. I believe that the SSD advantage is HUGE. I'd consider dropping to a lower end MBP 15" and add an SSD to it to remain within budget.

Hope it helped and good luck.

Perhaps this perspective will help you.... I work on a 2,000+ page sized projects under RealStudio on both a MacBook Pro 17" i7 and a maxed out MBA 13". The MBP17 has 8 GB of RAM and a Sandforce based SSD. The MBA is my portable development environment to keep me productive everywhere I travel.

Is the MBP 17 i7faster? Yes. It benchmarks about 60% faster than the MBA and compiles roughly that much more quickly. Basically, it is a very nice machine to develop on the Mac side and simultaneously run Windows VM test machines. Its 8 GB of RAM lets it do that quite well. When it had only 4 GB of RAM, switching between Mac and Windows VM environments sometimes included annoying beachballs.

With the 13 inch MBA, things are a bit slower. Subjectively it feels about the difference between driving 70 MPH vs 60 MPH. I know it benches slower than that, but the usability is surprisingly good. As a portable development machine it allows me to continue work without interruption and I don't feel it as being too lagardly. The maxed out MBA 13 allows me to do iterative writing, compilation and testing with under 10 second cycle times. That's fast enough to let me write and work without feeling like I am waiting for the machine. It feels very much like my old portable machine which was a 15 inch unibody 2.5 GHz core 2 duo which had an intel SSD. I know the 15 inch machine was a bit faster, but this MBA is a delight for moving around.

I find myself grabbing the MBA instead of the 17 inch machine when I just need to do a bit of work or browse. If I need to do heavy coding, the 17 inch is better, but both are pleasant writing environments.

So, portability and lightness has its merits. The MBA 13's screen resolution shows sufficient lines of source code to allow meaningful understanding of code segments. I don't think I could write was well with a vertically lower resolution.

Biggest lessons over the last few years have been

1. SSD's make a HUGE difference in machine performance. The OWC Sandforce drives are even nicer than the Intel SSD's due to their write speed and endurance.

2. More RAM is helpful IF you need to run simultaneous OSX and VM environments during testing. 4 GB is sufficient for single OS use, but is at the edge of what you need for OSX and VM Fusion.

Would I give up my 17 inch i7 and use ONLY the MBA? No. I want more speed and power in my main writing machine.

Would I go back to a 17 inch MBP as my portable instead of the MBA. No. I much prefer the portability of the MBA and its performance is adequate as a secondary development environment.