JFK is a flat rate (plus tolls and tip), LGA is on the meter, so you are at the mercy of traffic, unscrupulous drivers, etc.

I can understand wanting to save money if you are alone, but I'm thinking it "might" be less taxi fare to ride that Airporter bus to port Authority and go down the west side rather than to Grand Central and go down the east side? In any case, I believe GC, PA, and Fulton are all large enough subway stations that (barring out of service problems) they all should have escalators. I think PA puts you on the ACE line which drops right at the Millenium Hilton door at Cortlandt? (I'll have to double check that).

Just looked...the E from Port Authority to World Trade Center looks very quick and close on both ends.