Which Horizon? It doesn't matter because they will be in the middle of nowhere. ;)

Zero Dawn.

I may be wrong but I don't think it'll even be close, Zero Dawn will beat Forza horizon

This is even better than your last comparison.

Horizon Zero Dawn - more than 5 million units ! So... its very easy.

Horizon Zero dawn is a scifi themed open world western action rpg, with a few monster hunter elements. Its a story driven RPG.

Forza Horizons 3 is a racer.

Ive seen gameplay and trailers for Horizon Zero dawn.... cant say the same for Forza Horizons 3.
Zero Dawn looks amasing, like a breath of fresh air, its something new in a good way.
Forza is just forza.

I both hope and think Horizon Zero Dawn takes the win in sales.

Forza is a yearly racer, and unless Microsoft heavily bundles it it wont do more than 2 mil. If Horizon Zero Dawn can't beat that I'll consider it a flop.

Horizon.