On the XBOX ONE, the Master Chief colletion sold around ~2,000,000 in a ~11,000,000 userbase.

Do you think that a Gears of War collection can sell better, or at least close this number's?

Yes, id say the interest would be greater.

Nope.

I think both will match in sales.

No. Halo MCC will continue to sel and has potential of reaching 3-4m lifetime. I like Gears more, but i dont imagine a Gears collection could match that considering Halo is overall a bigger francise

I'm rooting for Gears.

Halo has gone down hill since Bungie stopped working on it, so maybe.
But then Gears is in the hands of a new developer as well so it could fall just as much.

I'd take Gears > Halo any day, but unfortunately i don't think it would.

if it aint broken out of the box then sure it will

I doubt it. Halo has much more brand power than GOW.