I don't think there's a difference, then. Using the Adodb class probably just gives you a much more user friendly interface to the database than using the odbc functions. And yes, I do understand that "more user friendly" is a relative term.
Part of the problem is that Microsoft's databases are only accessible as COM objects or via the ODBC or OLE DB interfaces. PHP doesn't have an OLE DB extension, nor do I know of one being developed. I don't think that Access is a robust enough database to merit the writing of an Access-specific extension, either. Most people who use a MS database for any sort of website are probably going to use SQL Server.
I suggest writing your PHP test site using a database abstraction layer, such as the PEAR DB wrappers. That way, you write your script just once. The interface to the database is the same regardless of which database you're actually using. That is, you make function calls on a generic DB object, not to something specific like MySQL, SQL Server, ODBC, or Postgres. Behind the scenes, you set up which database the generic functions will forward the calls to.
This means that you only have to write the code once, and should your database change, you just swap out the implementation files without touching a single line of the actual program code.
I think this gives you the most flexible and efficient mechanism for testing multiple databases with the same code. At least, in PHP.