You are not going to like a 9.9 on either of those. We have a 16ft version of the lund (beam is about 64" - looks like the current WC's are wider), older model equivalent to that, with a 25. Empty a 9.9 will get it to plane, but it is about worthless empty. Add another person and gear and you need a LOT more hp. I'd even say that 15hp is going to have a hard time. If it were me, I'd max out the hp on that boat. It should take a 25.
Also, don't discount the affect of having the right prop on there. That will make a big difference also. You may get the 15 to run adequately for your purposes if you have the right pitch prop, but I would imagine you will be pretty low on hp.
Our boat will do about 22-24mph with an 11 pitch prop and a good load. Empty and a 13 pitch prop it will get closer to 28-30, but the boat is useless without any gear.
Both of those options are going to be good boats. That having been said, for the small utility boats like that especially, a square chine hull and flat bottom are going to run better with less hp. When you have a more rounded bottom and round chine the boats draw more water and don't plane as high = less efficient. You will need more hp to go the same speed a square chine/flat bottom boat will do with less hp. A figure I have seen before is about a 20% difference in hp.
