I've been using the Geotrust seal for years because it's the cheapest. The only thing I don't like is the way the seal is a live entity. So if you display it as is on all your pages, you're relying on Geotrust's server to be up all the time. As soon as they are having server issues, your pages fail to load, getting hung up on the Geotrust seal. There is a way around this that I've been using. You put the seal by itself on it's own url - then use an iframe to call it. So when Geotrust is having server issues, pages still load normally, but you get a blank box where the seal should be.
I did see an article that tested which seal people have the most trust in - [baymard.com
...] (I didn't reread this, so take it with a grain of salt.) Norton wins, McAfee second, while Geotrust scores on the bottom. But I really think that has everything to do with name recognition. Everybody recognizes Norton. Nobody recognizes Geotrust. You would think Geotrust might score better with the live button that shows the name of the company and the exp date of the certificate. I could be wrong, but I think they're the only company that offers that. Despite being slightly annoying to administer, it is a good idea. I guess that shows that it really is all about name recognition.
I'm constantly getting bombarded by McAfee with emails and phone calls that I ignore. They mention the names of all my competitors that use them. They mention how studies prove that sales increase by x-percent as soon as you add the McAfee seal. I don't know. I wish I knew the real answer to that. One thing I can say is that a professional looking website probably does much more for trust than a security seal. One day, I made it a point to look out for security seals on ecom websites. I checked a scuba equipment site and noticed that they displayed no seals whatsoever. But the site was very professional. There was a ton going on. They offered lessons, classes, online articles. There was so much going on on the site, the last thing on my mind was "Hey, where's the security seal? I'm outta here!" I did notice that once I added an item to basket, then they showed the seal, on the Basket page and checkout. So that might be the answer. On sites that are either sparse or not so professional looking, a security seal might help. But on a great professional looking site with alot to offer, I doubt that the seal makes any difference. But either way, include the seal on any page that needs SSL.
Edit: Fixed link ending in period.