Ramjet and Afterburner are two different kinds of boosts available to racers in Star Wars: Galactic Racer. Both have their strengths, but they essentially do the same job.
That said, you will want to use the option that provides you with the maximum advantage during a race. Before deciding between them, it’s important to understand how they work and which is the better one.
Ramjet vs. Afterburner — Which one should you use in Star Wars: Galactic Racer
Both Ramjet and Afterburner have some similarities in terms of how they work. Their main purpose is the same: they provide a massive speed boost to your vehicle. Think of them like the nitro boosts we typically use in mainstream racing games. Both have finite uses, and once they run out, you will have to wait for them to refill over time.

While both of them increase speed, they are not exactly equal. If you are going for a pure speed boost, Ramjet is definitely better than Afterburner: The speed boost you get with Ramjet is significantly more. You can use your vehicle to ram into opponents using Ramjet, which is not allowed with Afterburner.
However, it’s not all rosy with Ramjet. While you get higher speed on one hand, it significantly increases your vehicle’s heat. If you do not focus on that, your vehicle can even explode during a race. There is no such worry with Afterburner. Your speed boost might be moderate, but your vehicle will never catch fire and blow up on its own.
If you are a beginner, my recommendation is Afterburner. You do not need the extra speed boost that Ramjet offers. You can easily do well with Afterburner enabled on your vehicle and win races. Once you are in the late-game stage and have vehicles that come with pulling upgrades, you can safely use Ramjet since there will be very little risk of explosions (unless you abuse the system).