If most of your colleague's meetings are in Oakland and Pleasanton, it would be worth locating near a BART station. San Jose is a challenge to reach by car or public transit, so I would not plan around it.
Assuming nightlife is not a consideration, the most central point I would recommend is Castro Valley. The Comfort Suites is rated well while the Quality Inn has mixed reviews. Both are next to the freeway and within walking distance of the Castro Valley BART Station--on the line that runs between Oakland and Pleasanton. Although Hayward is centrally located too, its better hotels are not centrally located.
If nightlife is a consideration, I recommend staying in Oakland. There are a couple Marriott properties near 12th Street BART Station, as well as a couple hotels near the waterfront. The waterfront hotels are better with a car, while the downtown hotels are better without a car.
One last property I would recommend is the San Leandro Marina Inn, which is in a peaceful area south of the Oakland Airport. I recommend this property only with a car, as bus service is somewhat limited and BART is not nearby.
P.S. Avoid the hotels near the airport; the area is not very appealing.