SIC Time
First of all, unless the SIC is required by the regs and/or type certificate of the airplane, it may not be loggable. You would just be along for the ride.
Second-in-command time is time in the logbook, but its value for climbing the career ladder is limited. PIC time is what you really want.
On the other hand, the experience, in and of itself, may be valuable, and you might make some good contacts. Developing a contact that could further your career may be far more valuable in the long run than the flight time.
Hope that helps.