I made a similar decision just a month ago. I chose the Prius (2008 with 73k, $13,600), and I'm 99.9% sure it was the right choice. A well-maintained Prius will easily last 300,000 miles, and the batteries are known for their remarkable non-failure rate. You can find a lot of sources saying that you're way more likely to replace the transmission in your Fiesta than the battery in your Prius in the next ten years. (I sold Ford cars... a Fiesta is NOT a long-term vehicle. They're really meant to be local commuter vehicles- essentially gas powered bikes. I'd be shocked if that car lasted you longer than the warranty without some major repairs.) And CPO is always a good way to go, as an unlikely battery failure will be covered under warranty for quite a few more miles. I've been getting 39-42 mpg in cold weather/city driving, and I've been able to get 50-60 mpg in warmer weather/less stop and start situations. The car has a surprising amount of space, too. Between the roof, trunk, and a hitch rack, you could fit a lot of shtuff in this car, and bigger people fit comfortably in the back seat. (As for the winter driving rumors... I've been doing just fine on my steep, slushy driveway. It's all in how you drive.) Negotiate that price though... you could probably get it for $12k or so.
In short: do it!!