Ellipticals offer a lower impact, full body workout. You can get a good elliptical for less than a good treadmill.
A good general knowledge article on Treadmills at Runner's World. Here are some rules of thumb from the article:
• "... getting a deck larger than the standard 60" x 20" probably won't matter."
• "... pay attention to the motor's continuous duty rating, rather than its peak duty rating. Runners need 2.5 to 3 horsepower continuous duty, but any more can be overkill."
• "Look for lifetime coverage of the frame, three years on all parts, and at least one year on labor. And since all machines will require some maintenance, be sure there is a certified technician in your area ..."
UPDATE: We went with the Schwinn Elliptical. It is fantastic!