I think that the biggest difference is cost - it is 25/pp roundtrip. Although I think that you can get the entire van for 130/roundtrip. In addition, you may stop at other locations before you get dropped off in order to drop off other passengers.

Here is a link:

http://www.airportexpress.com/

If you are trying to go the cheapest way, then taking the el train downtown is $2/pp. Not recommended though if you have more than a carry-on sized piece of luggage to contend with.