Depends on what type of niche you are dealing with from my experience. And if the site is doing financial transactions. 

I notice customers seem to be fond of SSL certs and security on eCommerce sites where they are dealing with transactions.

I notice a few sites seem to be using more http://privacy.truste.com/ CERT and what not, how big is impact on conversion I guess you need to test before and after implementation, I have not personally tested it. 

What type of transactions are you trying to secure?  

In my experience, the average consumer doesn't seem to care about 'who' the CA is - in fact, they don't seem know there is more than more.  All they know is that the icon in the address bar should turn green and stay green for the duration of the transaction. They also care that their browser doesn't provide an error or warning that the certificate is not valid.

I predominantly deal with GeoTrust certificates and have used QuickSSL and TrueDevID SSL.  Neither seems to impact on whether a consumer will purchase or not.  That's not been measured however. 

My recommendation is choose one the of main players (GeoTrust, Thawte, Truste etc) and spend the time to make sure your user experience is going to give you a better conversion.