I understand that 

    http://www.example.com/product
and
    
    http://www.example.com/product/

can actually be seen as 2 different URLs and is probably best avoided for SEO purposes, but how does this affect querystring parameters. Which is better (or even 'legal')?

    http://www.example.com/product?source=googleps
or
    
    http://www.example.com/product/?source=googleps

Do you need the trailing slash before the `?`