well the info that I am including is based on my experience working with my company for the past 4 years. I work as a mechanic and we operate Pratt & Whitney PT-6 engines on Turbo Beavers, Turbo Otters, Twin Otters, King Airs, and Beech 1900C and D. And TPE331 engines on a MU-2, C-441 Conquest, and BAe Jetstream 31.
From my experience the TPE331 engine that is computer controlled on our Conquest is a good engine, it is very easy to rig and doesn't give us any problems, I know the guys working on the MU-2 are not crazy about the installation of that engine, and they have had some problems with the engines on it over the past few years, but that could just be a fluke. As for the engines on the Jetstreams they are not computer controlled, so they have a whole bunch of control rods to operate the FCU and MFV, PPC, etc. Of the 8 engines we have on the Jetstreams we have never had more then 5 engines that were not loaners on the aircraft at one time since I have been working on them, from my experience we are always having to adjust the rigging on the aircraft, and in the summer the engines are always temping out so they only operate at between 10,000 and 17,000 ft as they can't get any higher, they require Water Methanol in the summer or they have no payload, these are comments from our pilots. I am assuming the reason for the bad performance and requirement to have water methanol is that British Aerospace just picked a underpowered version of this engine for this aircraft.
I love the Conquest as it will fly high and fast, but not the Jetstream.
I really can't say anything bad about our PT6 powered aircraft, other then yes it is a dirty installation with the exhaust at the front so you get exhaust all over the plane, and yes it does burn more fuel, but since our PT6 aircraft can get up to 25,000 ft, they end up burning the same amount of fuel as the Jetstream so there is no real savings there, and the PT6 equipped Beech 1900 flies at 290 Kts true from our pilots and the Jetstreams fly at 255 Kts true. The MU-2 it cruises around 260 and our Conquest with its upgrades flies at around 310 kts true.
The Beaver, Otter, and Twin Otter of course don't fly fast because they were not designed to fly fast, they are bush planes. But the King Air according to Raytheon website is no turtle, it is capable of flying at 295 Kts. true.
So from my position with the company I work for the engines in our Corporate aircraft the MU-2 and Conquest are good TPE 331 engines and are reliable, but the TPE331's in our Jetstreams are not.
The PT-6 engines have proven to be very reliable in the dead of winter in Western Canada, and also in the hot summer, on average of all these aircraft we operate the PT-6 engine is more reliable in our operation. Yes it costs more to buy, and to maintain it, but from my position as a mechanic, it has required a lot less maintenance.