About a year and a half ago, I was in a situation where I was flying pic jet for a pt91 operator, with the goal to get to the majors. I feel that you really do need to have a backround in 135 or 121 in order to make yourself marketable to the majors. There was no reason for me to go 121 so I can build SIC time for 3-5 years, when I could fly turbine pic 135, so I took a job as a captain flying jets 135. I am confident that I will be able to find a good job with a strong carrier even with zero 121 time. As stated above, turbine pic is the most valuable time that you can get, period. Since you have a good amount of 121 already, I wouldnt think that it would be a problem having your PIC time come from 91 flying...
In addition, taking a corporate job will open many doors for you. Think about the stability of the airlines today. You will be a captain on an extremely popular aircraft, and if in the future you get furloughed, you can always return to 91/135 and resume flying as a captain in a Kingair. I have a few type ratings that allow me to fly a total of 15 different models of corporate jets. Trust me, if in 10 years Im flying with a major and I get tossed to the curb, it will be infinately easier for me to find work outside of 121.
Take this for what it is, I have never worked for a 121 operator. But hopefully this give you something to think about. I would get that turbine PIC and market yourself as a captain, especially considering that you have the 121 experience.