the important thing about the $50 unlimited metro pcs plan is you have buy a phone from them, and right now you can't use it with an iPhone. you can't bring your own phone on this plan.
you might be able to buy a cheap android phone , and move the sim to your iPhone, but i haven't heard that this works yet
the other thing is you have to pay $5 for mobile hotspot
i don't see anything that says you get a lower speed.
I've heard metro pcs customers have their voice call compressed and t-mobile does not compress voice as much
but i do not know if this is still true. i heard you can tell if someone is using metro pcs by the voice quality