L'Aerobus is the cheapest ($25 round trip) and also the most time-consuming. The bus only stops at a few hotels downtown. For other hotels, you get dropped off at the downtown bus station while you wait for a smaller shuttle bus to take you to your hotel. That smaller shuttle bus ALSO has to make a bunch of stops.
And it's just as time-consuming going back to the airport since the bus must make a bunch of stops before actually leaving downtown. Then, of course, it must deal with traffic jams on the highway.
If you've got all day, take L'Aerobus. Otherwise, I suggest spending $35 each way and get a cab.