Keep in mind that there are two elements of emissions.

Basically all the smog related ones, for which the Toyota hybrids do incredibly well. You cannot really improve on these cars.

The other is carbon dioxide. You only reduce carbon dioxide by improving gas milage, or driving less. A car that gets twice the MPG of another car will emit one half the carbon dioxide of that other car.

So compare the cars in the class you intend to buy, my guess is the hybrids will win out on MPG.

George