Select the engine that best fits the model you are flying and how you want to fly it.
If you take two APC type propellers lets say an 11/7 and a 12/6
If you spin a 12/6 at 12,000 RPM it will require about 1.3 horsepower, it will produce around 8.5 pounds of static thrust with an estimated top speed of around 69 MPH
If you spin the 11/7 at 12,800 RPM it will also require about 1.3 horsepower, it's static thrust drops to around 7.9 pounds but it's estimated top speed will jump to 85 MPH.
This spread will become greater at the size difference in the prop gets greater for any given horsepower.
So the bottom line is if you like to go fast pick an engine that turns a smaller diameter at a higher rpm, if you like to hover and 3-D pick one that spins a larger diameter at a lower RPM.
The problem is most companies don't publish RPM's with different props, so places like this are the only real source of information and it tough to figure out.
You would not want to prop your ROSSI to turn a maximum of 12,000, and I don't care what you do with an LA your not going to get it to work at 17.500.
Read through the posts on the different engines and pay special attention the what prop and RPM numbers are being reported, do the math then pick the one that fits you the best.