This is an interesting question. I bring it up because of an article I read about cars that cost considerably less than a hybrid and rival a hybrid for gas mileage. There are standard gas vehicles that get 40mpg on the highway. There are cars in this group that have few materials that are difficult to recycle when their useable life ends, unlike the Lithium batteries in a hybrid. These items alone create a lot of back and forth discussion about which is better.

Lets first cover some raw info:
Basic and popular hybrid -
Prius = average of over 40 mpg $30,000 and up

Basic standard engine -
Fiesta = average of about 40 mpg $14,000 and up
Forte EX = average of 30 mpg $19,000 and up
Mini Cooper = average of 32 mpg $19,500 and up

Basic diesel engine -
Golf TDI = average of 36 mpg $24,000 and up

Clearly the hybrid wins in fuel efficiency. However, how do we measure environmental impact when the construction of the vehicle is so far out of touch for the consumer? Consider this, batteries are toxic waste, all cars have one but is it a small manageable battery with a quality recycle and rebuild program in place? What about the battery material, can it be reused?

Currently, Lithium batteries are the most common in a hybrid and the infrastructure to collect and recycle or rebuild is lacking. However, as they are used more, demand may help create this missing element. At the moment, I call a tie for the hybrid vs high mileage gasoline and diesel. I do not have sufficient evidence to really decide one way or the other. You will need to decide for yourself based on your needs. As long as you are making an effort to consider the environment with the decision, you are making a good one, in my humble opinion.

See the EPA website for more information.