I support the whole idea of ridesharing, which will really increase your per person mileage, and make the insight no doubt the more efficient choice. For flying, it is much harder to determine the actual efficiency. In the year 2000, the average airline efficiency was 38 passenger miles per gallon (PMPG), including takeoffs and landings. I Have not found the figure for 2005, but aircraft are newer and jet engines more modern, and occupancy levels higher - all of these affect PMPG. For example, the B757 - an old design, consumes on average about 50% more fuel than the A320. Both the new Boeing Dreamliner and teh A380 will provide another leap in efficiency in their respective size classes. While airborn, some airliners now reach 80 PMPG. However, another consideration is the number of takeoffs and landings, since takeoffs consume vast amounts of fuel. If you get a direct, non-stop flight from TX to NYC on a full jet of modern design, my guestimate is you might reach about 60 PMPG, about what you would get in your insight.
You could easily double that efficiency by sharing the ride.
Of course, either a commercial long-distance bus, or train, are much more efficient than either plane or Insight. Greyhound claims an average of 162 PMPG for their fleet as occupied and used.
Either way, sounds like a fun event in NYC!