Lurker:
O.K. You prefer a new car to a used one, which is understandable, but I would strongly recommend thinking about a year-old car, a demo or a so-called executive car.
These generally have anywhere from 3,000 to 12,000 miles on them - hardly broken in - and you don't have the big depreciation loss. The downside is that you can't get as much for a loan on a used car but - you still have the remainder of the factory warranty in most cases.
As for what car to buy; you need to do more research and the internet is the easiest place to do that.
There are a ton of car-test/report sites. One is Edmunds;
www.edmunds.com
I checked out a Hyundai Tiburon and they show a V-6 manual going for $18,500. MSRP. The site allows you to pick and chose options and you can also figure your trade-in value and insurance costs. You can do a comparison between the Mustang and the Tiburon in terms of cost, insurance and performance. For free.
These figures may be 'ball-park' but give you a base to work from. You can also check out the official Hyundai site and 'build' a Tiburon.
http://www.hyundaiusa.com/
Ford has a website, too at
http://www.ford.com where you can check out the new Mustang.
I can't see spending near $20,000. on a little Korean-made car that probably isn't anywhere near as well built and sophisticated as the Ford Mustang but do your homework and make your decision based on facts, not other people's opinions because everyone will tell you something different and on a Mustang website you're bound to get a majority opinion that will recommend buying a Mustang. Surprise!
That may ultimately be a good choice but insurance and gas mileage are a factor for you so maybe a V-6 'Stang would be better. As it's been stated; if you test drive a new Mustang, those asian 'sport' cars won't be half as attractive anymore.
Keep looking, use the internet and don't rush into anything. It's a lot of payments to make so get a good value and the car you really want to own, not a compromise that you'll end up hating a year later.
Best of luck to you.
Please let us know what you end up with.