We are going to go with a side console on our 1648 jon.
1st question.....we currently run a mercury 9.9 2 stroke....its a little powerhouse and pushes me 180 lbs and my wife 120lbs good enough. We fish the Juniata and Susquehanna mostly but also raystown, sayers dam, and a few other lakes. I found a 15 hp merc 4 stroke with controls for a pretty good price, I'm curious though will the 15 give me anymore top end speed considering its a 4 stroke? never owned a 4 stroke but I've been told they run better at lower speeds, which is mostly what we do on rivers considering its a prop and I don't want to tear it up on rocks.
2nd question....if I go bigger say 20 hp or 25hp....will it be much of a difference than the 9.9 in the depth I can run? right now I can probably run in about 16" of water.
I'm not familiar with the Mercs but usually the 4 stroke is heavier than a similar HP 2 stroke. Except for the weight I doubt you will see a difference in performance. 4 strokes are smother but I hear newer 2 strokes are just as smooth and as economical on gas.
There is a difference between a 10 and a 15 in performance. In most cases they are the same engine but the 15 has a higher top RPM and maybe a different prop. It will show on the water.
A 20 or 25 will make a big difference on a 16' boat compared to a 15hp.
If you go with a side console make sure you have power trim for the rivers especially the Susquehanna and the Juniata, THINK ROCKS!! If you don't have power trim you will have to get up from the console and trim the motor which is a pain in the butt. Power trim will handle your depth concerns.
I use a 15 HP tiller on a 14" john on the upper Susquehanna. Except in real fast water I never have a power problem. If the water is too fast I stay out of it.
Remember, 4 strokes are a lot heavier than two stroke outboards, The extra weight may make the 5 more hp a wash. I had a Merc 9.9 and it pushed my 14 ft jon boat with a 5.5 beam with three men as fast as I wanted it to. I would talk to merc dealer and asked him if you would get any negligible benefit from the 4 stroke.
When you are up to your butt in alligators, it is hard to remember your intent was to drain the swamp. Stay focused!
Your 9.9 2-stroke will have much greater torque out of hole to quickly get boat on plane vs a 15 4-stroke plus is lighter. I have a 25 2-stroke on my 16' semi-V Lund and it outperforms most 40-4 strokes plus has far less weight on transom. If at all possible....I'd stay with a 2 stroke. Personally for your size jon boat, I'd take care of that lil 9.9 & enjoy!!
Loggy and WW brought up weight. Why don't you get the specs so you know for sure what you are dealing with weight wise.
I have and have had Johnson and Evinrudes. I never had a Merc. I know with the Johnson and Evinrude 10HP you can increase the HP by changing the carb. I have been told you can also change the HP by just changing the jets in the carb. You should look into that. It may save some money.
One other question. Are you buying a boat with a console or adding a console to your boat? If adding a console that alone will add weight. Consoles take up space especially in small boats. Your current tiller takes up very little space.
I don't know about your questions 4 stroke is definitely heavier but I don't know if it is significant. We went from a 15 to a 25 (I mean 9.9 to 20...) when we upgraded on our 16 ft deep V Sylvan. The 15 slogged along. The 25 gets you on plane, but barely.
One other consideration is fumes. We do a lot of trolling and I don't miss the fumes from the old 2 stroke.