Is head-on or trailing wind better?
A tailwind provides a faster groundspeed.

The aircraft moves through the air so whether the ground is moving relative to the mass of air has no relevance to the aerodynamics of an aircraft at cruising altitude.


  a British Airways passenger jet approached supersonic speed this week as it rode a surging jet stream from New York to London.


from The Telegraph, 10 Jan 2015

They mean groundspeed - so the term supersonic is moot.


  The Boeing 777-200 jet reached a ground speed of 745mph as it rode winds of more than 200mph across the Atlantic. At ground level, the speed of sound is 761mph.


Aircraft often use the predominantly westerly† jetstream when flying from the USA to Europe, From Japan to California or across the USA. Flying the other way they pick an altitude to avoid it.
 Wikipedia

The Jetstream is a high speed, high altitude, wind that blows moderately reliably in one direction.

 Metcheck

† Westerly means "blowing from the west" - towards the east.