As a rule I take a car service to/from LGA so I don't have to deal with this sort of thing. The cost is not all that much different. If I have enough time I will at times do Super Shuttle but you need to know some of the highjinks they like to play (like picking you up for JFK and making you first go to LGA to drop off other paxs or trying to put 13 paxs in an 11 pax van). And only take Super Shuttle to the airport never from the airport.