Pete has given some good reasons for west to east. I'm choosing east to west for the following reasons:
- It allows an earlier start, avoiding the worst of the heat in the midwest and the worst of the cold in the high-altitude west.
- The earlier start affords me longer days (on average), centering the trip around the summer solstice.
- Since more of my riding is likely to be in the early morning than in the late afternoon, I avoid more of riding into the sun, and avoid more of having the drivers behind me blinded by the sun.
- I get the worst hills over with earlier, and save the best scenery for last.
- Donna Ikenberry's book asserts that east-to-west riders have a statistically higher probability of success. I think that might be mentioned somewhere on this ACA site as well.