Freewheels are just crap, they screw onto your hub and when they start to break, youll know about it cos you face palnt the ground alot cos they slip every time you pedal, one peice dirvers are just like a casette, the pawls are built into the hub and are just generally better, its worht the extra money.
cassettes all the way. nicer sounding, stronger, and instead better for brakeless because freewheels, (depending on how loud they are) just roll for miles, casettes kind of feels like theres something there, theres a bit of pressure if you get me, its just alot nicer.