In my opinion the MCX is for someone who wants to say they have a bigger engine. It is for the kind of person who asks the salesman how fast the boat is and how much HP it has.
I believe it has been discussed on here a few times that the 2 motors have a very similar torque curve. The MCX probably doesn't realize that extra 40hp until a high RPM?
If the boat with 400 hours looks like it has been absolutely babied the hours wouldn't turn me off. That being said that is 400 hours of people crawling all over the vinyl, bumping into docks, hitting rollers at higher speeds, etc... I think Unless the one with the MCX is in mint condition I would go for the one with the RTP(assuming it is in really good shape).
I think the 330 hours less will be better for resale than the MCX if you plan on owning the boat for 7 years or less. If you are going to keep the boat for a long time resale might not be that important to you.