I just don't know if a iPad air would be worth the 100 dollars more. I do like that it has 64 bit CPU. I just don't know.
That 64-bit CPU is not being utilized at its full potential because it requires minimum 4gb of RAM to show its true power. The iPad air & iPhone 5s only have 1gb of RAM. Right now it is a gimmick (future proofed for next model ihardware)
I've owned both an iPad 4 and an iPad air (that I'm currently using).
The iPad Air is lighter, the form factor is better and it's simply a device that you can hold for longer periods of time. Because you're upgrading from an iPad 2 this is important as the iPad 4 is heavier than your second generation device.
Now it's also true that the iPad 4 performs a bit better in everyday tasks. The 64bits Cpu found in the air is using more ram and you can't have a lot of tabs open. Your air will crash from time to time and multitasking will be limited, forcing you to reload your tabs and games. This is quite annoying and one of the biggest disappointment with the latest iPad. This is compensated by a faster cpu but still, Apple could have added more Ram...
Finally there's one thing that might surprise you with the air. The screen is... Different. When you tap it it feels less resistant and more "plastic". It's a bit weird at first but you'll get used to it eventually. That's one of the trade-off they had to make in order to get that bezel smaller.
I'd say go with the Air because it's future proof but personally I'd wait for this year's iPad which will hopefully have a lot more ram.