In this instance I would still keep Magento since that has an established database. Making a lot of changes isn't really something I like to do when things are going well.
On a new site, I weigh the client needs. I have not found a single commerce type plugin that does everything that needs to be done for a full functioning solution, but the major ecommerce plugins can cover most client needs and it is easier for most of them to use a single WordPress based solution than two separate solutions.
I still think there is a long way to go with the available plugins, but they are much better than they were even a couple of years ago. I think a big reason for this is there are more of them now. Just a couple of years ago there were only 2 or 3 semi solid options, and they were buggy at best. Now there are nearly a dozen solutions that are pretty solid and much less buggy.
I expect in a couple of years we will see several stand out options and that will really bridge the gap between plugins and stand alone solutions.