The single vs. multi arguement is really irrelevant in this case, just my opinion. You're talking about one of the most bullet-proof engines out there that's installed in both of these planes.

I think that the owners should get some demonstration flights arranged, check it out on the cost analysis side of it, and put two and two together. That's just what I think. The speeds are pretty close, not going to do you a ton of advantage except on those really long trips.