I've never flown a King Air, so I can't compare it. However, the Merlin is pretty fast (280kts) and efficient. It has long range, but if you fill the tanks you can't carry any payload, and who wants to go 1000+ miles in a turboprop anyway?
Runway performance is not great, especially hot and high. If you have any plans to go to hot and high airports, get one with water injection-- that can make a big difference.
Also, try to stay away from anything earlier than a IIIB-- the performance without dash 10 engines is a bit scary. Of course, they have probably put dash 10 engines on the old airplanes by now, so maybe this isn't an issue.
They are reliable airplanes as long as you have someone working on it who knows Merlins. There are a lot of quirks.
All in all I liked the Merlin quite a bit. However, the last time I flew one regularly was 10+ years ago, so take all of the above with a grain of salt...