Rocketman -
This is a great question. I can't give you an answer because I have no idea how the system would work but this is what I would **think** would be the answer. I look forward to hearing other's opinions on this.
Since gaining your ratings through hard work and dedication, then paying your **dues** instructing, and eventually flying cargo/freight and moving into the regionals seems to be the standard route to the majors they know what you have been doing. In this case - it would seem that -121 majors would expect you to have a lot of time in the -121 world and they understand that a majority of that time will be SIC. But at least you have been working within their system, doing the type of flying that they do, day in and day out, making decisions that -121 opeartors are expected to make.
If you were to gain a 1,000 hrs of PIC time in a King Air working for this military contractor and doing who knows what type of flying, under what type of conditions, then I would think that the major would be a lot less sure of your capabilities to work within THEIR system of flying.
Now if we were comparing 1,000 PIC to 2,000 SIC within similar -121 world companies then the answer would seem pretty sure which would be more desirable.
Good luck in your decision. Standby for some good advice!
USMCFLYR