Just my two cents worth...

Diesel: They cost somewhat more than standard gasoline vehicles (but less than a hybrid) and they get somewhat better fuel economy than a standard gasoline vehicle of equivalent size and power (but less than a hybrid). The major downside to diesel is that the vehicle emmissions are much dirtier than gasoline-burning cars. There are plenty vehicle choices, but not every fuel station carries diesel. Future technology, such as "clean" diesel and "bio" diesel may make these vehicles pollute less.

Hybrid: They cost much more ($3-$5k more) than a standard gsoline-only vehicle (this is the only major downside to this choice). They get better mileage than a diesel, and significantly better mileage than a conventional car (example: standard Civic gets about 30-33 MPG, but the hybrid Civic gets 45-50 MPG). Hybrids are very clean-burning (typically ULEV or PZEV rated). Tax breaks can offset part of the hybrid premium. Future technology, such as a breakthrough in high-density, lightweight batteries, may make these cars less expensive.

I'm sure you'll get lots of opinions, however!

Bottom line: If you simply want to consume less fuel (always a noble goal), diesel is okay. If you want to consume less fuel AND pollute less, hybrid technology is the way to go.