I own both. I have a 2012 13 Macbook Air and a late 2011 13 Inch Macbook Pro.
I like using the MB Air when I'm travelling since it's light and can fit my backpack perfectly. It's pretty fast too.
For long term use however, I prefer my 13 Inch Macbook Pro for it's RAM and Hard Drive upgrade options.
Don't get me wrong - you can upgrade the flash drive in your MB Air if you need more space - RAM however, you're stuck with what you bought.
In my 13 Inch Macbook Pro - I have my RAM maxed out at 16GB and my original hard drive, replaced by a 240GB Kingston Hyper X - now, I have a very fast Macbook Pro.
As for how long it will last - it all depends on how you use it. If you don' drop it or bang it around - either one will last you long enough. I just sold my Original 2008 Macbook Air and it was still in mint shape and battery still last a few hours.
Too add - I also still have my late 2008 Macbook Uni - all I did was maxed out the RAM to 8GB and installed a 120GB Sandisk Extreme SSD - now it's pretty fast too.
If you don't need the superdrive and future upgrade - go with the Macbook Air - it's light and has a better screen.
But if you want to be able to upgrade the RAM and Hard Drive in the future and you need the superdrive - Then them 13 Inch Macbook Pro is for you.
Good luck - and if you're not happy with your first purchase - you can always return it and exchange it within 14 days of your purchase date.