TL;DR: Both the STP1-11 and Sellita sw300 are high-quality Swiss movements. However, the STP1-11 boasts a longer power reserve, better accuracy, an extra jewel, and superior quality finishing, making it the superior choice for watch enthusiasts.
The STP1-11 movement uses a longer and thinner mainspring compared to the Sellita sw300, resulting in a boosted power reserve of 44 hours. The Sellita sw300 has a power reserve of 38 hours. While the difference may not be significant, the STP1-11 provides an extra quarter of a day of power reserve, making it a better choice for watch enthusiasts who don't wear their watches every day.
According to Caliber Corner, the STP1-11 has an accuracy of -3s/day to +7s/day. On the other hand, the Sellita sw300 has an accuracy of -5s/day to +10s/day. While both movements are highly accurate, the STP1-11 is slightly more accurate than the Sellita sw300. Additionally, as per research done by WatchGuy, on the wrist, the STP1-11 would result in an on-wrist performance of +/- 2 to 4 seconds, which is significantly more accurate than the Sellita sw300. Therefore, the STP1-11 is the better choice for those who value accuracy in their watches.
The STP1-11 boasts 26 jewels, which is one jewel more than the Sellita sw300. This extra jewel is used to support the top of the barrel arbor, thus prolonging its longevity as the barrel bridge can easily wear out. The Sellita sw300 only has 25 jewels. Therefore, the STP1-11 is a better choice for those who value longevity in their watches.
Quality & Finishing
The quality of a movement can often be derived from the finishing. The STP1-11 can compete with the best in its class. All movements by STP come with pearlage, a Glucydur balance wheel, and Côte de Genève finishing on the rotor. The Côte de Genève decoration cannot be found on the standard Sellita sw300. Furthermore, all markings on the STP1-11 are better executed than the Sellita sw300, as per WatchGuy's findings following a microscopic comparison. Therefore, the STP1-11 is a better choice for those who value high-quality finishing in their watches.
While both the STP1-11 and Sellita sw300 are excellent Swiss movements, the STP1-11 proves to be the better choice for watch enthusiasts who value a longer power reserve, better accuracy, an extra jewel, and superior quality finishing. The STP1-11 provides an extra 6 hours of power reserve, is slightly more accurate, has an extra jewel for improved longevity, and superior quality finishing. The Sellita sw300 is also an excellent choice, but for those who seek the best of the best, the STP1-11 is the way to go.