Do small differences in fuel and speed justify spending a lot more for a 800 series 737 over a 400?
Are there any pax benefits to be gained?
Depends, does the added range and extra seats in the 737-NG series matter to you?
Are you currently leasing your 737 Classics or will you be leasing or purchasing them?
Fuel can make a difference, so can maintenance costs and speed differences if any.
if the NG allows you to serve a few more airports so can expand your route network, why not?