I was an 11 year 91/135 guy that made the move about 4 years ago to 121. I had friends that went to the airline side so I started picking their brains a bit and realized the 121 side was gonna be best long term.
So far it has been.
The best part of 121 is you can be whatever type of pilot you want to be. If you want to work hard one month, you can do that, if you want to take it easy, have at it. You can call in sick and never hear anything about it. Its why we have a sick bank. We don't have to ask for raises, or vacation. You can trade, swap, do whatever you want with your schedule. There are limitations but compared to a 135 company its night and day.
We work in TFP here at SWA (1.149 X Hourly rate of a normal airline) If you don't know what that is, search it out here its been discussed at length.
We also have line bidding here. Research that a bit, but its pretty good at the conflict drops that can happen and you make more money to do less work. Airline dream amirite?
Ask questions here, we can answer, we have a ton of us who have done the 91/135/91K side to 121 or WN.