Hi,If you have the money to spend the elliptical. They provide a better overall workout since they include arms, get you standing, and can also use reverse running which can work your body in a completely different manner. However you can't go cheap with a elliptical. You are looking at $500 for a decent sturdy model, and that's real entry level. If trying to do HIIT which i recommend then you might need to spend a little more, those machines if not high quality will not be great for going all out as fast as you can when needed.
If money is tight then i would go for a bike, still a great way for cardio, you can spend a lil less and still get a nice sturdy model, and are great for HIIT workouts.
So it basically just comes down to the money you can spend and what type of cardio you plan on doing. I personally still can't afford a good quality elliptical so i opted for the bike. Thanks