I believe the best setup depends on what car and track you are planning to race. High displacement engines, or high revving engines tend to be more powerfull with a high RPM turbo. These engines need more air volume to operate, and high RPM turbos are able to provide a higher air pressure, compared to the other turbo options (the game doesn't describe it specifically this way, but we can presume the high RPM turbo is a high compression one). It takes a bit longer to get the full power, the bigger a turbo is, the more mass it has, so more energy is needed to move it. A anti-lag system can help you with that. This setup is great on tracks with long straights (Sarthe, Monza, SPA, or ovals for example), where you are at full throttle for most of the lap, and the engine is more likely to be at higher RPM's. In this condition you will benefit from a high RPM turbo, making more power, compared to the other ones.
The low RPM turbo could be used with the opposite strategy, on technical, low speed tracks (Tsukuba), where you won't be constantly at full throtlle. You will need a smaller turbo that can spool faster, making early and linear-ish power. It also makes the car more predictable and easier to drive.
The medium RPM is the balanced one, my favorite option for most of the tracks in the game.