IMO, schedule flights would be easier and cheaper. With the charter you will be paying for reposition costs of the 737 and crew costs if the crew stays with you. If the crew does not stay with you, you will pay for the reposition costs of flying the plane back to its scheduled route, then pay again to have the plane fly to pick you up and again after the flight is done for another reposition flight.
Group sales are the best option. You may have to break the group up for the best deal since flights are book up to a year in advance (in other words, there are already tickets sold on the scheduled flights you will be looking at).
An example of our group's flight in 1996, we had 400 people that went from HNL-STL-BWI/DCA/IAD...drove from Washington DC to NY, then JFK-MCO on TW, then MCO-MSY on SW, then MSY-STL-HNL on TW.
The HNL-STL the group was on the same flight (747), at that point we were split onto four flights to get to Washington DC (MD-80s). Then we were again split on two flights to get to MCO (L1011s), and four flights to get to MSY (737s). Back on TW, we were split onto 4 flights over two days from MSY-STL (Md-80s), and split onto two STL-HNL flights over two days (747s). We paid per person about $875 for this entire routing. This was in 1996 and was a pretty good deal at that time.
Hope that helps!