I've done both.
I'd rather do the 121 job. Coronachan has kind of screwed everything up.
L3 was the most stable, but keep in mind, a lot of those contracts go POOF overnight. Pre L3, I was always "I wonder if I have a job in 6 months" as we went into contract silly season. And seeing people run off because they don't get along with a site lead or lead pilot.
I remember a Lead Pilot (M. H) trying to get me fired because I had my crew rest interrupted twice, while already on a high flight time waiver, and my room was literally broken into by a bunch of Afghans while I was sleeping. Yeah, you ain't going back to sleep after THAT. (Crew was there to "clean the AC" allegedly)
Woken up out of a dead sleep by a bunch of people speaking Durka, IN MY ROOM, and tries to fire me for a "I am too tired to fly safely" call. Oh, and I did that hours before launch, it wasn't like I showed up to the brief and went "nah, not flying".
It's a good job, but it's not as stable over the long term as people would like to think either. For better or worse, there isn't senority, and there is no union. If a manager don't like you, you stand a good chance of being fired over some violation of a rule you didn't know existed. (and was likely made up by the manager). If there are cut backs, good chance the more expensive guys get let go too, even though they have been there for years. (Talking your PICs who have been there a decade, that aren't IPs and don't want to be)