Get the most you can afford. When did you last hear someone say "I wish I had less HP." HP is much more than piston or stroke size. When I went to Charlotte Motor Speedway for a NASCAR rescue class we drove over to the gocart track. They had to start with a stock 5 HP B&S or Honda motor. Taking out the restrictor plate and changing to racing pistons and valves (for strength, same size as stock) gave them 60 HP. They did 60 MPH on the straight stretches of the gocart track. Same for the Cummins 5.9 when we wrote specs for an ambulance. Same motor, electronics and fuel injection changes gave it a 175-275 HP range in a Freighliner business class chassis depending on GVWR and application.