For $600 you'll have to find a solid deal and be careful to research quality, etc.... For $1200 you shouldn't have too much trouble getting something good.
As far as treadmill vs. eliptical, it's really personaly opinion so I wouldn't take too much stock in the one is better than the other comments. What's the best is what you'll use. We prefer treadmill. My feet fall asleep on elipticals and I hate it!