FT Staff + regular per diem vs. FT staff + agency per diem????

I'm moving to Cali next year. I'm trying to decide whether to travel, get a FT staff position, etc.

Specifically I'm interested in hearing from people who hold either

1. full time staff position PLUS a regular per diem position, or
2. full time staff position PLUS a per diem position through an agency.

How much do you make per shift? Do you get killed at tax time?

Right now I'm a full time ICU staffer at one hospital, and have in the past worked per diem shifts at another hospital in the city, where I pulled in about $400 for each per diem shift worked. Curious about how this compares to Cali Bay area per diem rates!!