I don't know the suzuki's well enough to give you a side-by-side comparison. That said, I do know the Yamaha's. I have a 1548 war eagle, with an older 25 hp 4 stroke Yamaha. Couple years ago I bought a 15hp 4 stroke to help my dad out since it was electric start and his old 9.9 JohnnyRude was giving him fits starting it with the manual starter, especially when cold. Anyway, before I was to give it to him, I had to make sure it was in tip-top shape and reliable. Thus, I did some work on it and then clamped it to my 1548 to run it. To my surprise, it ran within 4 mph of the 25hp. The difference, however, was VERY apparent in the time it took to get on plane. About 10 seconds. The 25hp is on plane in about 3 seconds. That's only 7 seconds difference but let me tell you, it's an eternity in certain situations. Keep in mind that the war eagle has a lot of "lift" built into the hull, thus it will plane easier than a lot of boats it's same size and weight. Hull weight without the motor (just the hull) is 353 lbs, plus the trolling motor (39 lb) plus the battery (41 lb), and me.
The 20hp is obviously 5 hp more than a 15, but, I personally think that if you'll be using is on a heavier 1648+, that you may not like it too much if you find yourself loading it heavy or running in current. In my case, the 15hp just didn't have the torque that the 25 does, mainly because the engine is MUCH smaller on the 15/20hp (they are the exact same yamaha outboard with minor differences). I haven't run a 20 much, yet, though...just enough to know that it's a sweet little motor but I couldn't compare it like I did with the 25 and 15. Most of the 20hp motors that I've rigged on customer boats were on 1542's and they all say the same thing, that they wish it had a little more torque to get on plane when running a little heavy, say, decoys, dog, hunting buddy, shells, guns, etc. Some of the older folks who spend a lot of time idling/trolling absolutely love them.