I would have loved and elliptical, but my husband has to keep his run times down for the APFT, so we got a treadmill. We also have a rowing machine which is FANTASTIC for full-body workouts IF you use the correct form. If you don't do it right, it's worthless. My husband did crew for his university, so he obviously knows the right form. Some day, I'll have him teach me. :)
The obvious advantage to the elliptical is the low-impact. When I was in college, I ALWAYS tried to grab one when I went to the gym (as did nearly every other girl, so sometimes it didn't work out!) Any aerobic exercise (cardio) will help all areas.
As far as muscle tone in the midsection, try to do core training exercises. I tried to look at core exercise equipment but there is just WAY too much to muddle though. Looks like it would be better to decide on what exercises you want, then get the equipment required for those. Best part about the core exercises is that most of them require minimal equipment, too!