Hi, I have many years experience in all Merlin's and if we're talking about a straight SWIII, I'd say, don't even consider it, this model is a very complex and user unfriendly, the batteries are too small and the engines are tricky to start, you should own a GPU if you operate this machine, it's not at all suitable for a grass strip, just my opinion, on a good day you will see 260 KTS, but the people you choose to maintain it will always be happy to see you!
The 90 will not eat your lunch when you go to maintenance, the engines are easy to maintain, landing gear is made for grass strips and the speed is not much slower than that of the Merlin, air conditioning is bad on the Merlin, nonexistant on the ground. The 90 not good in turbulence due to the low wing loading, but the best machine of the two you mention.
I doubt if you have MU-2 time or you would probably already know that the Mitz is made to do this job, a Solataire will cruise in the range of 330 KTS and is specifically made for unimproved strips, wing loading the same as a LR23, great in turbulence and low on maintenance, you must have previous experience to get insurance though.
Cheers, D.L.