Just bought my MS 30 Pilot II - 2003 model a few weeks back. There are significant differences and similarities - in the two designs. I have seen both. Both are well built boats, hands down better than some of the other better known production boats in the 30 ft class.(Have owned a couple of them - never go back !!)
Earlier models had more of a displacement hull, deeper draft, more of a hull speed cruiser - hence the smaller engine. 2003 and later were redesigned with a semi planning hull, and a tunnel for shallow draft.
The 315 hp Yanmar will easily put it on a plane but will be burning 6-7 gallons or more per hour above 2750 rpm - 3400 rpm = 25 knots. At hull speed she will run all day < 2 gal per hr.
I was concerned with noise. The little Yanmar seems to struggle and makes more noise than the big 6 - most of it coming from the side exhaust port - not the engine proper. At quick cruise it's no worse than any other diesel I've run.
My boat has the factory five blade prop - re-pitched to make rated rpm. I'm not seeing any resonant or sympathetic vibrations in the running gear, although there are reports out there in both designs.
The earlier boats with the round bottom do tend to roll more but answer a little better to the helm at maneuvering speed. The rudder on the later boats is a known deficiency. Several owners I've talked with have made rudder mods with varying degrees of success. In following, some what heavy seas >4ft one fellow described the handling as "evil"!! I'm looking at a "Thistle" rudder mod that is reported to tame this evil woman !!! But anything over 1500 rpm brings the rudder firmly in to play. (I fly a V tailed Bonanza - so I'm used to having no rudder).
So far I'm happy with the boat - lot of folks on here with various Mainship models in the 30 to 40 ft class. Most seem to be happy with them. Check out Marlow-Mainship thread in the Builders section - on the front page for more specific model discussions.
Welcome and good luck with your search.