I would have to say that the S76C+ would be hard to beat. For that matter, the A++ is hard to beat. Put those 1S1 engines in the A airframe, with no increase in allowable max gross weight, and you have loads of power. With C+s, you have a degree of commonality of parts, and both pilots and mechanics already familiar with the basic aircraft. It should be cheaper all around, rather than toss out all the spares and experience you alreadyi have.
The AB139 might be viable, but it hasn't been around long enough to tell.