Without a doubt, I'd go with Flight Express. You will be paid much more money, and you will be logging PIC time.
Don't count on the upgrade at Great Lakes being even 1.5 yrs. There's no upward movement there (where the heck are the capts going to go in the next year??). Upgrades could (and I bet will) come to a grinding halt.
Turbine time looks good, but PIC time is very valuable and will set you apart from the thousands of low time pilots out there with a few hundred hrs of turbine SIC. Also, if you're looking to go into corp/charter, you'll make a lot more connections on the FBO side of things.