I used to be a hugh fan of Johnson! Than I bought my boat and have a 150 Ocean Runner on it and have many, many problems. I don't really blam the motor for the first or second rebuild within one year (I think it was how the previous owner took care of it). It does make me leary of OMC motors (Johnson/Evinrude). When I researched my kicker I went with the 4-stroke 8HP high thrust power trim/tilt Yamaha. I love that motor and it has no problem with current or wind on a 23'6" alum boat. At the time that I was looking at motors I talked to many about all different brands. I was told to stay away from OMC...going through bankruptcy. I know they are now owned by bombarder, but I still love that Yamaha.
I can tell you one thing- with any single cylinder you will get quite a bit of vibration over other two cylinders just because it is easier to balance two cyl vs one cyl. I don't want to have you shy away from Johnson but when I upgrade my big motor I will go with a Yamaha. I think you would be safe with the smaller Johnson's but I would go with an eight horse instead of the six. It is better to have too much horse power than too little and since both will troll down to what you need the 8 is a safer bet. If I owned a drift boat I would definately go with the 8 hp yamaha. It is geared down to go slower and push with the same thrust as a 20 hp motor. it will not push the boat too fast for trolling but it will not get hindered in stronger current or heavy wind.
With either motor (6 or 8, yamaha or johnson), you will still need to get it up in the r's after trolling with it all day just to burn off excess carbon build-up. Any marine mechanic will tell you that. They are built to troll but they still build up carbon deposit if not run at higher r's.
If you look around and see the power thrust honda, I would choose the yamaha over it. The honda is not geared down, all they did was change the prop. I don't remember where you are from but give pacific boatland a call. They sell johnson/evinrude, yamaha, and honda. They are also an excellent company when it comes to customer service. They want to make sure you are happy with your choice. They are a little higher but the motor comes tuned up ready to run not just pulled out of the box and handed to you like some other comanies in the area.
I hope this helps you and gives you a couple of options. I hate to buy something and not be informed about all the options on the market so I like to pass on the info to others.