When you’ve got Office 2010 64-bit installed you can only install the 64-bit version of SharePoint Designer.
Sadly this version has some integration issues.
Main ones I’ve noticed is not being able to use the “Edit in SharePoint designer” link in the context menu and not being able to edit page layouts in SharePoint Designer.
64-bit Office also has some integration issues. So unless you need 64 bit for working with really large Excel Sheets, I would stick with 32-bit for now.