Well I've got 733s and 734s on my fleet and chose them after much consideration. As per my calculations, compared to the 737NGs, I pay about 100k more per month per aircraft in fuel (when fuel at $900), but the leasing cost for the older models is about 60% of the NGs and I save a minimum of 300k per month by leasing the older aircraft (leasing savings only). Overall, the economics is better or at most on par with the newer models.
The real problem I'm having is getting a higher LF. Most of the routes I fly have high competition with supply much greater than the route demand (on average, the good routes have 1.5-2x the supply) and the increasing RI is not increasing my market share on many routes. Also being a relatively new airline, my flexibility with cash is very limited and I can't make any major fleet changes without going bankrupt. I've stopped spending for the short term and am just waiting for my weekly transactions to stabilise to get an idea of my actual income. Hopefully things should improve in a while with increasing RI and CI.