with a cassette you can run even smaller rear sprockets so you can get it even lighter (you get to loose a fair few chain links too). Also, the "freewheel" mechanisum tents to be better on a cassette cos the paws are wider and closer to the middle of the hub. Having said that, if its a crappy cassette then a normal screw on freewheel might be better, esp if you dont wanna run a small gearing anyway.