I have decided that is generally best to go with the maximum rating of the boat. However, I did stick with a 150 as opposed to the 175 when I bought my Nitro Z-18. The difference between a 115 and 150 will be more significant. I have owned a new 18' metal boat powered with a 115 Merc. 2 stroke. The 150 Mercury engine will not have to work so hard and you will likely be very happy with the fuel mileage that you will see. If you fish alone most of the time, and not fully loaded, you may be fine with the 115 and the right Propeller matched but you will be slower all the time- probably low to mid 40's full throttle, at best, in normal conditions. That might not matter depending on your situation. Also, I think a boat generally sells a little better later in life if it's carrying the full HP rating. The 115 will be more economical and weigh less on the boat. If you are buying a new boat, don't downscale everything to save a few dollars. Of course, anything you don't finance will save you money in the long run. I would say the last thing you want to do is buy a new boat and then not be happy with it's performance because you didn't spend a few extra dollars up front. Your question is a bit loaded since we don't know all the variables you are considering with its use. This is just one guys opinion about this- really, if you get you a new boat and you are out there fishing and enjoying it- that's what counts!