I've done both. 14 years at NJA and now 3 years at my ACMI. It was very difficult to leave NJA but the lack of career progression was a serious downer. That being said the overall QOL at the fracs was decent and the benefits were good but for me I find I really enjoy the ACMI lifestyle. I'm making more at year 3 pay than I was at year 14 at NJA. The layovers are far longer and more fun at my ACMI gig.
The schedule is not for everyone but it works very well for me. Flying a 747 all over the world beats flying a small private jet to Teterboro every day of the week. It probably doesn't need to be said (but I'll say it anyway) that flying cargo is simpler than flying pax, particularly pax that are always used to getting whatever they want. This is my first cargo job and I'm kicking myself for not making the switch sooner. To each their own but I have no regrets in leaving the fracs.