Welcome aboard Ed. That can be a rough decision but I guess it all boils down to how far you want to travel and how fast you want to go when you get there and what type fishing you do. I recently downgraded to a 9.9 as a smaller rig is easier for this ole man to handle when fishing alone. I can fish fresh water anywhere the bigger boats can just not as fast. I just spent a week at Buggs Island and 3 of us fished out of the boat with no problems. There were a few times I was missing my 150 I use to have when I got to thinking about other places on the lake. But we managed. The only other times I can think of when a bigger motor would be nice is when sudden storms pop up and you need to get off the water in a hurry. Another reason I went with a 9.9 is so I could fish the Suffolk lakes without having to just rely on a trolling motor. Also there are trolling motor only lakes around where it doesn't matter what size you have since you can't use it anyway. You can still fish the lakes but most require you take the prop off or some just unplug the gas line.
We have some great people here and I'm sure others will jump in and give their opinion.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va