I definitely recommend the MN to 125th st and then transfer to the M60 bus. It generally doesn't take more than half hour. Alternatively, I'd still get off at Harlem and pick up a cab...usually black livery...not many yellow cabs up there...not sure what they'll charge, but likely not more the $20, including the Triboro toll. The cab ride would be 10-20 minutes max depending on traffic...so you'd save 10 minutes on the MN plus at least half hour on the bus from GC to LGA.