Use MacBook Air or Pro for app development? I need to replace my old Mac and I am wondering which one should I get between:

MacBook Air or MacBook Pro

The primary goal will be to develop iPhone and iPad Apps and OSX Widgets.

Is there a better one I should get for these purposes?
This is a very difficult question to answer, it depends so much on what sort of development you are doing, even if you will have access to an external monitor or not.

Really, what I think you are asking is if an Air is capable of running Xcode for minor development tasks.  As if it isn't, you know that the Pro would be.

Certainly an Air is capable of running XCode, but on the smaller screens it's not going to be a comfortable experience as XCode does enjoy vast amounts of screen real estate if it can get it.

Also, the lack of a full size keyboard may be annoying when coding on a laptop keyboard, whether it's the Air or the Pro, I bought an external keyboard simply to have access to full size keys for forward delete, home/end etc etc.

So you may find that a Mini, with external kb/monitor may be your best alternative, depending on your financial limits etc.