Here is a recent article with information about both old and recent studies regarding user trust of seals, although I'm not sure that's quite what you're looking for.
However, as for choosing one or the other, you should definitely prioritize the SSL cert. Generally a seal simply indicates that a third-party company has scanned your website for any known security holes, but it does nothing to actually secure your website. An SSL cert actually encrypts the data the customer sends to you, and this actual encryption is much more important than any symbolic seal, regardless of the trust factor.
Furthermore, I'm not sure that most security seals will even allow you to pass a scan and display the seal if you don't have an SSL cert already. (Of course, if you're only talking about signaling, you can always just swipe a seal as an image, but that would be deceptive and possibly illegal depending on your jurisdiction.)