I'm not sure that you can describe wsdl creation as top-down vs. bottom-up. I think it just depends on what tools you're using. Some products lend themselves better to schema creation first. Others are better in starting with code. It really depends on what you have access to.
I think you may be able to describe an overall SOA implementation strategy as top-down vs. bottom-up. Bottom up is start building services at the data access level and then higher order services on top of those. Where top-down is get a major project, create a few services that are to be leveraged, scrub, rinse & repeat.
In this case, bottom up is the way to go. The ROI is longer, but it is because those services continue to be leveraged that you experience the savings soa purports to enable.