Steve, I can give a little first hand experience, and some general 115, 90 info.
First off, nice choice suzuki's are my personal favorite. My buddy's and I run them on backtrollers, so that trolling speed thing is on our minds. On my own boats, I've had a 90 honda and f115 yamaha on the same boat. The 90 was a bit more fishable in the exact conditions than the 115. That particular 90 had 2.33 gears, the yamaha has 2.15. This is important as I think it was largely the reason the 90 was a pinch better. I've ran the df140 suzuki on the same boat, and it is better than either in every catagory, including trolling speed. 2.59 gears on all of these, df90, df115, df140. Of note, the df115 is a lot closer in power to the 140 than the 140 is to any of the 150's. Same block, 2mm larger bore and 200 rpm more redline is the only difference. I don't know exactly what boat your putting it on but I would not hesitate at all, if you can stand the $$, to get the 115. Especially cause we're talking about suzuki's. The 90 is a great engine too if it will push it ok. Having said all that, if it's flat calm, none of these are slow enough except for cranks. For the most part we use our vantage right up until the wind is too much then use the big motor. I don't feel it limits how many fish I catch. good luck