I heard that starting the plane from a cold & dark state at the gate isn't very realistic, so which one do you guys use, the short or long state when flying the pmdg 737 or 777? and what is the difference between the 2?

Short is more realistic. In short, by and large, the aircraft has the been electrically powered up, and pneumatics are available. APU will be running and online. Or Ground Power/equipment will be supplying the same. I would venture that in real life ops, this is what greets the crew when they arrive at the A/C. Ground/maintenance crew generally set up from "cold and dark" or "long" states to hand the aircraft to the crew in the "short" state

Thanks