I currently have a 2006 25HP Merc 4 stroke on my boat, app. 30 hours of operating time. I am thinking of upgrading to a 50HP, the max my boat can support according to the manufacture. If I go with a trade what is a "fair" trade-in value I should look for or expect? Am I better off staying with the 4 stroke or should I look at 2 strokes as well?