Undergrad student MVNO or Contract phone

Hi! I'll be moving to Japan this September to attend my undergraduate course for four years however I'm slightly stuck about the phone/internet over there. My dorm probably also has wifi as its a student dorm so if im indoors I wont be using it much

Abit of background info -
I'm currently looking for a new phone whether its going to be the samsung galaxy s7 edge or iphone7 I have yet to decide.

I heard that MVNOs are a lot cheaper but also a lot slower?
I'm not exactly on a tight budget but i've heard that contracts can go up to 10k yen per month while MVNOS are about 3k yen?

are MVNOs worth the hype or should i go for a contract?

Do a search here. It is true in general that MVNOs offer quite lousy data speeds, but there are exceptions.