Re: rounded pontoons
There are three end designs:
Rounded
Cone
Slant
Rounded are a little labor intensive, but, third world production so cheap. They have the least amount of drag as a design.
Cone designs are the easiest to produce as well as being friendly to PVC fabric limitations. Drag is usually marginal.
Slanted designs provide higher stern buoyancy, but at the cost of additional drag.
The trade-off in these three types is drag versus buoyancy. Depending on all of the other variables such as engine weight, boat size, etc., one end design might be better than another on an otherwise identical boat. In general though, the heavier the engine the better off you would be with more buoyancy. The lighter the engine, the better off you would be with less drag.
So, the answer to the original question is....It all depends on the individual boat, the individual engine and how it is loaded.
I wouldn't worry about cone design unless you plan to buy a very expensive boat that you plan to keep for 15 or more years.