The Best of Both Worlds
I've found that I prefer my MacBook's trackpad for working in iPhoto and Aperture, and that I feel much more comfortable with my iMac's Magic Mouse for Illustrator and InDesign. So I bought a Magic Mouse for the MacBook and a Magic Trackpad for the iMac, and now I'm really happy -- I can work the same way whether at home or on the road.
The Magic Mouse gives me 3-4 months of use on ordinary alkaline batteries and (like any optical mouse) becomes erratic when dust or cat fur builds up on the sensor. A quick puff into the sensor, and it operates properly again. On rare occurrences (maybe once a month), the mouse gets a funny feel to it when the computer comes out of sleep mode, and this is easily resolved by turning the mouse off and back on. It's by far the best mouse I've ever used.