Headz 2 had more extended licensing muscle both in acquiring tracks and in the release itself.
Headz came as a more directed shot acrossed the bow for I think a great many people as an introduction into abstract, down tempo into that pesky term trip hop. Jumping off the more polished point of Royalties Overdue and graphically this time 3D of Massive Attack in what I'm guessing is a respects due effort on James part. To pay homage to one of the godfathers of the street impacted graphic work we now see so so much of.
With this James neatly presented the still developing components of the down tempo sound in its varying shades and degrees from the american roots and the originally completely overlooked but by the 94 release of Headz; huge Tranquility Bass they came in peace. Early "trip hop" and earlier Bleep/Bass pioneers Nightmares on Wax, Autechre with a complete nod to detroit and electro, Palm Skin as we have come to discover, dj Shadow with two brilliant early examples of his work and more.

the 2nd compilation on mo' wax is its' most famous as it opened the ears of people to those who experimented w/ hip-hop, and pushed the boundaries further.