I live in DFW and was commuting 40 miles each way. That got me out of my Jeep Cherokee and into a Civic sedan. My average mileage after 18,700 miles is 29 MPG.
Are you going to make payments on your vehicle? Interest on $28k versus $20k will kill your better MPG and your tax credit.
Don't make your decision based on a tax credit. If after all your math is complete and you are coming out ahead, then factor in the tax credit as a bonus to your research. But, I would not let it enter into the purchasing decision.
Have you been to
PriusChat.com? I spent a couple months over there while on the Prius waiting list. I learned a lot and somebody had a good spreadsheet that you could use to compare the total cost of Car A versus a Prius. Note: I've been looking for it but can't find it. I actually created my own and deleted it not 5 days ago. Talk about timing. It's how I determined that I could pay X amount of dollars in a car payment and save X amount of dollars in gas payment. At some point buying a hybrid (or in my case even a regular Civic) made economic sense. Also, I like driving a cleaner car, emissions my friend. :)
--edit--
I am heavy footed and get one of the lower scores in MPG. Your "mileage" may vary. :) Also, many of the Prius drivers look at it more like a game, because of the display showing Gas and Electric consumption/regeneration. With that display I would see it being much easier to drive efficiently than a regular Civic (I'm not sure if the HCH has a similar display).