I'd get the Macbook, no question. (if you don't mind the small screen)
Now I will share my powerbook experience...
I have a 3 year old powerbook, that i have used almost every single day for the last 3 years. it still runs amazingly well, runs LEOPARD amazingly well, and does everything I need it to do. I told myself recently that i'll get a new one when this one just doesn't run anymore. I just don't see the need to upgrade for myself and I'm a graphic designer too!
The quality of the powerbooks was wonderful. Can't be beat in my eyes. It's the best computer i've ever owned. That said, I had the screen start to die this summer, which could happen to any notebook. I called apple, they sent me a box, I sent it in, and had it back in 3 days with a brand new screen. It was like having a new machine...so that said...
I will say this...BUY APPLECARE...whatever you do...BUY APPLECARE...
Cover the computer for a full 3 years and don't worry about it...you will be much happier when you don't have to spend 500+ dollars for a screen replacement or something. I know I was.