Air screen size 11 or 13 in
Pro screen size 13 or 15 in
Air has no DVD drive
Pro has a DVD drive unless it it's the retina display then no DVD.
Air has no hard drive but uses either 128 gb or 256 gb flash drive for storage.
Pro has either 128 gb or 256 gb flash or 500 gb or 750 gb hard drive. So this comes down to how much you need to keep on the Mac.
Air is very thin and lite
Pro is little thicker and heavier. But still thin and lite.
Air only comes in the intel I5 1.3 ghz processor
Pro comes in I5 or I7 2.3-2.9 ghz processor.
So it all comes down to how much you have to spend and what you will be using it for.
I have one of each and they both are great machines.
Btw these specs are based on the current models not the ones just announced.