All modern saws will run between 12,000 - 14,000 RPM. The difference is in displacement, which means more hp and torque. The bigger saws will hold max rpms (don't bog down), use bigger bars/chains, and once you get up to the bigger saws, are capable of using more aggressive chains and sprockets. That's why they cut faster. It's easy to get talked into a buying a big saw. If a 3 hp saw is good, then a 4 hp saw is better, and a 5 hp saw is awesome. After all, you can't have too much power, right? Well, I personally would not recommend someone jumping from an MS 180, which is just under 2 hp to a Stihl 400 series or Hq (X)70 series with 5+ hp. These are useful, but very dangerous tools, and if you don't have the experience, these things can cause alot of damage to skin and bone. The fact that you're asking this leads me to believe you are somewhat a novice, so I would say stick with a Stihl 025/ms250, or husky 34(X) - 350 . They're all around 3 hp, and will pull a 16" chain through about anything, and will handle a 20" a little slower if you need them to. Then, once you have mastered using the medium power saws, you can move up to the beasts if you think you need to.