XML file encoding format "utf-8" VS "UTF-8"?

When the XML file was convert to ASCII. It is different values for user at the three characters of utf and UTF.

<?xml version="1.0" encoding="utf-8"?>


<?xml version="1.0" encoding="UTF-8"?>

I tried to create a new xml file with vs2005. utf-8 form file generated by default.

which one is a more standard definition? thanks.