Flight Sim is also used in certified trainers in the UK. I've logged several hours in an FNPTII powered by FltSim.
I use both, FS when I'm on Vatsim and X-plane for fun/curiosity. The idea that X-Plane has better dynamics is not true. It's a complete myth, largely propogated by Mr X-Plane. The sim community at large acknowledges that the dynamics in FS are just as good - but limited by the aircraft designer. You can't add arbitary station loads and custom moments of inertia in X-Plane.
X-Plane derives it's flight model from the visual model using "blade elements" entirely which in some ways is good, but in others is bad. It's simply not possible on a PC to model the huge number of polygons needed to fully model even a simple aircraft with blades. FS flight dynamics are only as good as the model but in theory at least, you can provide a better flight model than X-Plane although the work involved is significant.
Both sims fall short in extreme conditions of flight and near the edges of the envelope, although I would guess that even the beefiest PC would struggle to model an inverted flat spin - especially as eye candy also has to be included. That said, I spend 90% of my time in big iron driven by the FMC so I don't really care about eye candy outside the cockpit or purism of flight models. I fly a PSS777 merge most the time and it has been custom tweaked to fly as real as possible, within a few percent of real world performance in terms of climb, speed, fuel burn etc. In fact, performance is a big weakness of the default X-Plane aircraft - you need addons to get realism. The fuel burns, glide rates etc are terrible in the defaults - even MS did a better job here and they are not good either. RealAir, PMDG pretty rule the roost, especially for systems realism - there is nothing, on any PC sim, that beats PMDG for VATSIM airways flying although I'm looking out for v2 of the Airsimmer bus series ;-) I would say PSS but the way they handle transitions from the runway to the start of the SID can be pretty weird!
At the end of the day, a religious debate and entirely down to personal preferences but absolutely untrue that MS is "inferior" unless you base the argument on subjective rather than objective measures.
Sim Pilot Magazine - Flight Dynamics: FSX and X-Plane battle it out
Flight Simulator Guide :: Articles :: X-Plane vs. Microsoft Flight Simulator
xplane versus fs10 - Page 3 - FlightSim.Com Forums
Sorry,forgot to answer the question. Or rather, ask one in return.
The Just Flight 300 Professional is excellent although for systems realism, it is a little weak. It does have an FMC and TCAS but the comms and overhead are the usual "almost there" implementations. The external model and flight dynamics are top. But that doesn't help, I don't know of a good 400 model.
Maybe you could consider a merge with a 400 panel and a freeware 400 visual model. That would give you 2 Dash 8s in your hangar but you would lose the realism of the flight model in the 400 but if your aim is to practice your scan and procedures, then something like this might help...
http://www.arch-spada.com/panels/dash8400.jpg
Cheers