Your choice of a PMDG aircraft was the right first step. The realism they provide is second to none in my opinion, so your sure not to be disappointed.
Your choice of aircraft should be driven by the type of flying you want to do. For me it is the JS41 because I enjoy the high work rate in the cockpit and, as it is designed as a regional carrier, it is best suited to the shorter hops, typically airtime between 45 - 120 minutes. Manual control of the throttles in all phases of the flight means to me that there is more ''flying'' with this bird, and very short VFR hops are an option as well. The other difference is that the Jetstream can get into airfields that the 777 can not even consider, so this in itself opens up a world of opportunities.
If on the other hand you want to fly high and fast, and travel to diverse destinations carrying 100's of passengers then opt for the 777. As the guys have already pointed out these two birds offer a completely different experience. I suspect most people passionate about flight simulation have a variety of planes in their stable, and that is the beauty of flight simulation, you can get both experiences over time, which one you start with is dependant on your personal preferences. Happy flying