Flying wild
I think you are more or less spot on with your comparison between Jet2 and bmibaby, however Jet2 face some challenges going forward that other airline who have invested in newer aircraft are less exposed to, the recent ruling against Jet2 over compensation for aircraft faults inevitably increases the cost risk for delays.
For much of the bread and butter work down to the mainland Costa's the 737-300 is still a very good work horse, but needs more and more TLC as time goes on and whilst Jet2 clearly pull all the stops out when something goes wrong more and more passengers are less tolerant of delays, they expect aircraft to preform like new cars where you seldom if ever look under the bonnet.
Fuel costs for older aircraft are a straight trade of against a much lower lease or ownership/purchase cost.
There is probably oversupply of seats in Jet2 home turf of Northern England and moves by Ryanair to become more customer and family friendly will overtime eat into Jet2 market and FR have cost base of less than half of that of Jet2.
On the plus side they now have a large loyal customer base in Leeds,Manchester and the Midlands, my only worry would be that number of mainly minor tech issues like loss of cabin pressure,RTO's, evacuations, diverts,increase and that one of them get badly handled by the crew, but a steady flow of newer aircraft overtime should address that