You can now vote for the German 'Half-Blood Prince' cover art on the Carlsen Verlag (German publisher's) website.

Note: Both covers contain SPOILERS.

To view the artwork and vote, go here.

To vote, click on the cover of your choice. (Clicking a cover will not give you a larger version, as some might expect.)

My vote went to the first considering the second would appear to depict Harry twice.

Source: The Leaky Cauldron