The sharks off Guadalupe are mostly sub-adult to young adult. Sizes range from really young (a buddy of mine said they saw one that was about 4 to 5 feet, and looked like it was still trying to learn how to swim) to about 16 or 17 feet. Most are in the 13 to 15 foot range. They primarily feed on fish, though fur seal and sea lion feeding events do occur there.
The sharks you will see (if, indeed, you see anything) off the Farallons are true adult sharks. Diet is pretty much exclusively marine mammals, and sizes range from about 14 feet or so to about 19 to 20 feet (very rare.)
Interestingly, Lawrence Groth (who you'll probably meet if you're going through Golden Gate/Great White Adventures/Shark Diving Int'l) found that chumming works great in Guadalupe, and does absolutely nothing in the Farallons. Conversely, towing a surfboard gets hits up here, and wastes gas in Guadalupe.
Make sure you sign up early; the Farallon trips sell out pretty quick.