It's all a crapshoot these days. In the last hiring boom of the late 90's, PIC time was golden, prop or jet didn't matter. But there were several hired even without PIC.
As for what the majors will want next hiring boom, everyone has a theory, but nobody really knows.
A PIC, prop or jet, has to be the decision maker several times daily. The judgement and experience of being a PIC cannot be trained. Some airline recruiters say that because they hire future captains, they want people who have shown the ability to be a successful captain.
However, jet flying is very different than Beech 1900 flying. I ride upfront on mainline 737s quite frequently and am amazed at how much flying is done through the FMS. It seems that enroute, it is all done there. As a 1900 driver, that is very foreign to me. Some airline recruiters may want people who have more experience in operating jet aircraft in that manner.
I certainly don't know myself what the best course of action is. But I would guess that you still want PIC time ASAP. Go where you can get it as fast as possible. If that is in a Beech 1900, then perhaps after you have 1500-2000 hours, if you don't look to have any prospects for major airline jobs, then maybe look at getting some larger jet/swept wing/FMS time to enhance your resume.
But then again, if I really knew what I was talking about, I would have been flying for a major airline a long time ago.