Like anything else, it depends. There are good and bad jobs/companies in both.
I've worked at one ACMI (and one pseudo foreign ACMI/scheduled cargo), and interviewed at a top fractional (and wasn't hired).
The best, most alluring part of the fractional position was the known days off/on for an entire year. That was awesome. However, it did come at the cost of eight on. six off. And to me, that's not a schedule I wanted to commit to for my career.
I loved the variety of places the ACMI jobs took me to, and the long layovers that were often like vacations (a week in Guam, hiking in the mountains of Kyrgyzstan, strolling through Ginza in Tokyo, spending a day exploring Berlin). That said, it is exhausting and fatigue is sometimes an understatement. You lose track of how many time zones you've crossed, what time it is, and when you need to sleep. Some people cope better than others. However, it will absolutely age you faster and has proven to be bad for your health... but it is fun.
If had to chose between the two, at this point in my life, I'd be more concerned with the stability, retirement, health coverage and fall back opportunities.
You can jumpseat on many ACMI carriers. Might want to give a whirl to get the feel.