All the buses go in the same direction, so it doesn't make any difference where you get off the bus. However, I think you can walk from the DL Shuttle to the rental cars, so that might be faster. The Marine Air Terminal is the last stop at LGA, so it won't save that much time.

It's been a while since I've taken the NY Airport bus from Manhattan. I use public transit because it's much cheaper, runs more frequently, and takes just as long (a traffic jam at the Midtown Tunnel at night?!!!)

From midtown Manhattan, it takes me about one hour on the 4/5 train to 125th St. and then the M60 bus to LGA in the early afternoon.

Between concourse B and C (or C and D, or both, I forget), there are some rocking chairs where you can rock and watch planes. You get a much better view if you go upstairs.