I think it all depends on the promotional content.
If you give a great teaser that shows almost everything and is enough for the visitor to jerk of, he will use it instead of clicking. That's the risk with videos.
The thing is just to tease.

I really think that it can depend on one picture, one video cut : the look of a bottom to his top, a particular face expression, the view of a perfect ass, etc... If the visitor is captivated by a particular image, he will go to the sponsor site. Now too options : Either he pays the membership (if the site looks serious, is not too expensive, etc...), either he goes to torrent sites to find it free.

So i use my own judgement. If i find an image that teases me a lot, i use it as preview. On the contrary, if some pics in a gallery are disappointing (one actor looked good on one particular image, but is ugly on others), i just remove them. If the most teasing part is in the video, i use it. If the video shows too much, i don't use it. If the video shows boring bored actors, i don't use it. You got the point. I select everyting that is positive, disregarding the negative elements, or the too positive ones ^^

Regarding deeplinks, once again it depends. If the deep page is well one, it surely helps. But it's a risk, because it usually only shows one part of the site. The best is deeplinks that give a direct access to the scene the visitor is interested in, but don't forget to add previews of the other scenes of the site. If it doesn't, the visitor may leave the site without seeing the whole content. So i think that sending to a home that shows a lot of previews is good too because the customer's eye can be caught by new images. And if it happens, he will be more willing to get a membership. If the scene you promote is one of the latest updates, it will be on the homepage anyway. And if the visitor is really motivated, he will look for it and in the process will browse the site thoroughly, which is great.

To sum up. Everything depends of the promotional content. Focus on the teasing part, delete the bad one.

Too much free content is the enemy. The thing is to tease, not to make the visitor jerk off on your site.

Ideally the link has to go to a page that shows both the scene and all the content the sponsor site offers. That's why i don't usually link to TGP/MGP pages, they are too restrictive. A good deeplink is for example the ones from Suite703. They show a teaser and samples of others scenes below. The homepage is great too.
I like Lucas Entertainement video teasers, especially the interview or behind the scenes ones. So i find them usefull because the vistor wants to know more, without seeing too much.

Hope it was helpfull. Of course, i don't always follow these rules myself, must admit i'm often lazy or not motivated by some content