Lots of questions there, let's take them one at a time.
You're talking about basically 2 hypothetical boats. A .21 has been shoehorned into a few Miss Bud's but no .45's placed into a SW 36 that I know of yet but I suppose that someone is trying it. The choppy water that you are throwing into the mix makes a difference with the Bud as they don't like rough water. Take into consideration the particular powerplant that would be inastalled in either boat, a sport engine or full-tilt race engine. See all of the variables that we have just so far?
The Aqua Jet is an okay boat but I wouldn't put a .21 in it, the plastic hull is just too weak.
The Fun Cruiser. Here's where you get my honest 2 cents and my opinion only just like my signature says. I feel that the Prather boats are way overpriced for what you get.