* "Off-the-shelf" scanner-based certifications are not held in uniformly high esteem by companies with IT security groups.
* HackerSafe just won the Pwnie award at Black Hat for "Lamest Vendor Response".
* Talking about how you have a security SDLC, code reviews, and secure architecture will offer much bigger warm fuzzies to customers.
Also, avoid blanket statements like "so the data is secure". Marketing 101. If you're going to invite objections, make sure they're objections you want to deal with, not ones where you'll eventually have to concede.
Also the little green badge does slightly improve conversion rates for marginal and otherwise unknown ecommerce sites.
So Hackersafe can be some good marketing, even if it's rather a joke from the standpoint of what they check for (if you fail their scan, you really fail; passing tells you little).
Having the customer host a complex web application is likely going to be less secure then a hosted solution.
Consider the customer has to manage the code, applications that support the code (mysql, apache), manage the web server, and manage the physical security of the web server.
Your data is safer hosted.