As far as the new line, Apple just released their new operating system, Lion (as you may have heard), and with it, their new line of MacBook Air's, so the new laptops won't be coming out for another year.
As for endurance, you're not going to have to worry about that with an apple computer. My first Apple computer was the PowerBook G4, the model back from 7 years ago. I only JUST got my second apple computer two weeks ago with the release of lion, and i finally gave the PowerBook away. Apple computers will stay in good shape as long as you keep them in good shape. When you start eating while you're using your computer, or you start to get careless and drop it once or twice, you're decreasing its life by months at a time. Just be careful and no matter which computer you get, you'll be good.
For the photo editing, if it's in your budget, i would suggest getting Photoshop. It's the most professional photo editor out there and it works like a dream on a Mac.
From the sound of it, you're not going to be using that much CPU, or actually utilizing the full potential of either of the computer's hard drives, so I would recommend the MacBook Air. Of course, if you're going to be storing a lot of music, movies, and photos on the computer you might want to pump the hard drive (you'd have to purchase the computer online to customize it like that, but it's probably better to purchase it online for a host of reasons). The only problem with the Air though, is that you're not going to have a CD drive, which could be a bit of a bummer if you want to watch a movie. But other than the lack of peripherals, it sounds like the Air is the computer for you!