One thing to consider about fuel efficiency: do you really expect the cost of gas to go down? I don't. For one thing, on a policy level we should be working to include externalities in the pump price. For another thing, by choosing a less efficient car now, you're going to pay more over the lifetime in operating costs.
To do a simple calculation:
10,000 miles driven in a year (this is a low estimate)
price of gas at $4/gal
avg. 20mpg = 500 gallons = $2,000/yr
avg. 28mpg = 357 gallons = $1430/yr
And I'll be willing to be the cadillac won't get 20mpg.
I've mentioned this before, but the best car all around that I ever owned - considering efficiency, cost of maintenance, reliability, operating costs, usefulness - 1993 Subaru Legacy wagon. 1.8L 4cyl engine, manual transmission, nothing fancy.
And it's the nothing fancy that really helps keep the costs down. Not much can go wrong because there's not much that *can* go wrong.
We got rid of it because it didn't have air bags and we were going to have a kid.
Here's an ebay search you might find useful for comparing to the Pontiac you were looking at. Ford's quality has been excellent since the turn of the century.
http://www.ebay.com/sch/Cars-Trucks...00&_sadis=200&_fspt=1&_nkw=Ford+Fusion&_sop=2