I like the idea of calling up manufacturers to see what their most popular styles and colors are in terms of sales, that may be one of the closest indicators of what's "best."
One other thing I wanted to note was that, since it is a persuasive essay, don't forget to explain why your control pond is ideal to be a control pond in order to determine the best soft plastic. The best persuasive essays consider and shut down counterarguments and considering this topic, I think one big one to address will have to be the the external validity of the experiment that you're conducting on your own since you are restricted to one body of water, in a particular region, with its own combination of life in it, ... etc etc.
Unless your teacher for the course is also an avid angler with a good understanding of the sport, you shouldn't have too much of a problem with this area. I also took AP English in high school and wrote an article on catch and release that ended up in the local paper. Writing about something you really enjoy and care about makes writing more enjoyable for both you and the reader because your passion will show through your work! Good luck and have fun with it!