With that mission profile either aircraft will do the job.
The Merlin 300 was the last version of the Merlin series built before the line was shut down in 1986. It’s a fine aircraft with a wide operational envelope. It is faster than the B200, cruising around 290kts, and has substancially more range than the B200, around 2,300 miles. The Merlin burns less fuel per hour than the King Air and has better pressurization with a max diff of 7.0 and has a larger cabin and more baggage space. On our Merlin III we regularly put 4-5 sets of golf clubs in the nose compartment alone, it almost seems to be designed specifically for golf bags.
The Garrett 331’s on the Merlin are cheaper to overhaul and the P&W -42’s on the King Air, and with the 331’s TBO of 5,400 hrs vrs. the King Air’s 3,600 hours the per hour engine reserve is substancially less.
Now the downsides of the Merlin. Fairchild increased the MTOW on the Merlin IIIC and Merlin 300 to accomodate the increased empty weight that was packed on as the Merlin evolved. The earlier models had an empty weight that was 600-700 lbs lighter. Due to this increased MTOW the Merlin 300 requires a type rating where the King Air does not. While parts are easily available for both aircraft (King Air parts are more expensive), it is much easier to find a qualified shop for the King Air than it is for the Merlin. If I knew where you were based I could tell you the closest facility. Furthermore, the Merlin is not exactly what you would call a “short field” aircraft. If your missions involve shorter (less than 4,000ft.) or poor quality runways the King Air is by far the better choice.
The B200 is one of the rare aircraft that may not lead the pack in any one catagory but it does EVERYTHING well. They have been a pillar in the Pt.135 charter business for years because of their reliability, versatility, and their servicability. You can get it worked on just about anywhere and they are incredibly docile and easy to fly.
Downsides of the B200 are the shorter TBO coupled with more expensive overhauls. While they are reliable and have a stellar dispatch rate, they are still very expensive to maintain. There are a large number of life limited components and calender or time predicated inspections to comply with that can be pretty spendy that simply don’t exist with the Merlin. And lastly (and most obvious) the price of admission for a decent B200 is almost double what you can get a good Merlin for.
In closing, both aircraft will do the job for you. The Merlin will fly faster, farther, has a bigger cabin, and will burn less fuel than the King Air. It is much cheaper to buy and cheaper to own but it has eccentric needs. The Merlin needs a maintenance shop that understands the aircraft or you are going to spend a lot of money financing someones learning curve, and that is never a fun proposition. But if it works in your situation and there is support near by the Merlin is a hell of a bang for the buck.
If it doesn’t work for you than go with the King Air all the way. It’s a damn fine aircraft as well.
If there are any other questions you can send me a message and I will send you our info. We have 25+ years of experience with Merlin series aircraft.