The pros of being in Form 6.
1. Since STPM is rated among the hardest examinations in the world, so you can be more ready for your tertiary education. The most obvious example is STPM v Matriculation.

2. It is worldly recognised and same level as A-Level. So you have chance to enter local universities unlike A-level (very tiny chance).

Caution
In US Education system, they don't have pre-U. After their students completed their high school, they will sit for SAT (Student Applitude Test) then apply for university. So if you plan to further your education in the United States, think properly because although you can transfer your STPM to US, but the credit hours are limited.