I suppose the regional jet "boom" is coming to end and turboprops will be more common again. It is a clear fact that even newest small regional jets simply can't achieve as low operating costs as turboprops on shorter sectors. In fact, turboprop market has shown already signs of better future. Many regional jet operators, especially in the US can blame only themself of the current poor economical situation, as they got rid off very economical turboprops and acquired new ERJ145/CRJ200s few years ago just to notice that those good old turboprops were much more economical.
I have experiences as a ground crew of ATRs and Dash 8Q400s... In my opinion ATR and especially the new -500 series is a little bit better product than Q400. The ATR is a real workhorse, extremely reliable even in harsh winter conditions (-200, at least) and very economical. The biggest advantage of especially ATR72 is that it has two cargo compartments. If the flight seems to be tail heavy you can easily put some cargo/baggages to front cargo compartment to achieve better C/G and stay inside the envelope.
In case of Q400 the biggest issue is with payload and C/G limits. It is quite long aircraft and the only cargo compartment is in the back. This means that there is huge moment arm and you are easily out of the acceptable C/G envelope. If the plane is full it is hard or almost impossible to get all the luggages in due the C/G restrictions. You can't even think about to put some mail or cargo in there!
The other disadvantage of Q400 is the APU... it is perhaps the most unreliable APU what I have seen. Therefore many airlines use rather ground power as power shortages from APU are too common. The Q400 had a lot of technical problems earlier and airlines blamed that Bombardier was too slow to correct those quirks and offered too little help for the airlines... Same thing was with CRJ700. However, Bombardier has now achieved to fix most of the problems and reliability is far better, but I'm not too impressed even today of Q400 reliability especially in winter conditions... The advantages of Dash are slightly better range and cruise speed.
As a pax point of view, I prefer ATR72-500 over DHC8 Q400 anytime altough both are good aircrafts. Slightly less vibration and noise in ATR cabin.
I'm also under impression that the operating costs of ATR72-500 are lower and it is slightly more economical than Q400, but I could be wrong...?
Best Regards,
-AM-