It partly depends on your content. If you've a site of content that's no different from 20 other sites then offer download. If you have content that is so great the surfers will sign up whether it's streamed or downloaded.
The main thing most suffer from is their content is not exclusive, it's just a another scene that's replicated on a dozen or even a hundred other sites. So why should a member stay in a site that offer the same content as all the others in it's format, niche and style when he can only stream it?
He has to watch it at the time he can get to a computer, he can't save his favourite scenes to watch later and the quality is dictated by his connection and your connection and servers.
And all this to view content that is on 12 or 120 other sites. OK different girl and different sofa, but the myth that is exclusive has been totally discredited.
Of course if your content is that good you will not need to bother streaming it, they will be signing up in droves.
Over the last year I have seen lots of techie or traffic or legal solutions to the drop in our market. Most of them pie in the sky and some even worse. The problem is simple, the product we offer the surfer is not good enough to tempt him back to buying porn from us. If we can fix that we can turn the tide. No other solution will work.
I'm offering live shows and from the first week I think it's doing well. Yes it needs more design work and possible more daily shows, but I've been busy recently and will be away more in the coming weeks. If you run a site think long and hard about how you get more people to willingly sign up and stay rather than a techie solution.