The letters are arbitrary. If you keep an eye on the letters when previewing upgrades or infusions they may stay the same but turn blue or red to imply improvement or reduction. When trying to decide, don't look at the letters, look at the added damage for your build. In this case for example, you can go to MugenMonkey and see that with 40/40 the +10 Regular version has an AR of 414 and the Refined version has 424. It's an improvement, but a pathetic one. Sharp also yields miserable results in a quality build, resulting in a score of 429. If you don't level up strength though, the Sharp infusion is clearly the best option (409 AR vs 359 in Regular).