kingdee,
Not entirely pot luck about incuring a delay. You've chosen to fly with an airline that has overall a much younger fleet. All of Jet2's fleet is very old, not enough engineers to cover those aging aircraft (especially the 757's which are manpower intensive). There is no flexability in Jet2's fleet there's no spare aircraft at any time of the day(not so sure about Monarch).
But Playing the odds you've made the safe bet!