Depends on when you are going and how much time you have. The TransAmerica is a nice ride and if you will be going in the hot part of the year it will be cooler. We had a lot of 100+ days on the TA last year, but it probably would have been 115 or 120 on the Western Express.
The TransAmerica will take you MUCH farther up the coast than SF. The ride down the coast is nice if you have the time though. If time is tight and you have to get to SF you could take the train.