What's the difference between 'attachment before' and 'block' in views?
Block - separate views display that can be outputted in any region of the site.
Attachment - views display that can be outputted in addition to another views display, so you can attach this display to output of another views display.

Attachment has some special options like Inherit contextual filters:Yes, Inherit exposed filters:No, so you could use one exposed filters form for both displays.