I would'nt go with just streaming, it puts a lot of potential customers off. If i can't download the video and play it whenever i want, i certainly wouldnt join the site.
Streaming also provides a lot of other issues, such as always having the bandwidth to stream the files in real time to all your current members. If its streaming and going slow, then the scene will keep stopping/pausing etc etc. Also if a customer watches a stream twice, then you have used up 2xBandwidth vs a download link.
There's not a lot of successfull sites that only stream there scenes and theres a good reason for it.