I'm an easyjet guy who turned a 777 job down. Madness! I here you say. It was of course a tough decision which caused a few sleepless nights. So far I have not regretted it but ask me in another 5 years and you may get a different answer.
The main reason for me was age. I would have been 37 on joining BA and even with the increase in retirement age I would have spent more time in the bottom half of seniority lists than the top half. Money was of course an issue, long haul FO versus Easyjet Captain. The salary difference is significant and whilst it would gradually reduce over time and eventually become significantly more at BA it would have taken a very long time for gross earnings at BA to catch up and equal my Easyjet Command gross earnings.
If any easyjet guys are approaching command you would be surprised at the pay difference over the first 10 years. Have a look at some of the published pay scales for BA/Easy use your best guess for a BA command date and sector pay and add it all up year for year until they equal each other.
Also, factor in pension contributions over your career. The loss of the BA final salary pension was significant.