So I hear they change "form 6" to "Persediaan Pra-university" or simply "PPU".

STPM - if you get good grade or CGPA you can straight get your place in degree course at any university, inside or outside Malaysia. But fierce competition is fierce though~

Matriculation (except for IIUM Matriculation/Center of Foundation Study - CENFOS) - if you get lower then you expected, you may end up taking diploma at any local U. But if you're good, probably you can skip your degree and take master's instead! (I don't know this even a myth or real).

My suggestion, take STPM if you're not confident enough with your SPM results. Take matriculation if you think you want to test your interest before going to any university...