The short answer is that SubSonic is going to make it a lot easier to support the different databases with much less work.
If you look at the mojoPortal.Data.Common project, it uses SubSonic. Next look at code in WebStore.Data.MySql and you'll see how for most of its methods it just calls a corresponding method in mojoPortal.Data.Common and only a few methods need a db specific implementation. Also if you look in dbLinks.cs for all the data layers, they all use the same code from mojoPortal.Data.Common.
It makes the DAL code easier to implement, more simple and easier to maintain.
Another nice feature is what they call the scaffold page. If you are using the latest svn code, set this in your user.config (or Web.config)
<add key="DisableDBAdminTool" value="false" />
<add key="ShowProviderListInDBAdminTool" value="true" />
<add key="EnableDeveloperMenuInAdminMenu" value="true" />
you'll see a new link on the Administration Menu for "Developer Tools", click that and you'll see a link for Data Administration which is the scaffold page.