I'm from eastern SD and have ridden both places.
First I rode the horns for 2 days. Thought it was awesome. Had a local that knows the horns real good show us around. There was good climbs as well as boondocking.
BUT,,,,,,,,,,,
Later I was driving through laramie with some sleds on the trailer. We decided to grab a hotel and take a spin up above greenrock, with out guides, just some trail maps.
WOW!!! There is so much, so close to the parking lot in the snowies. I have been back in the snowies several times since, but have not went back to the horns.
If you can go on the drop of hat, the horns is great after fresh snow.
But if you are putting in for time off and have to take it or lose the trip,
I think the chances of you finding good snow is better in the snowies.
And all the variety close to the rig makes it worth the extra 2 hrs on the road!
1 last thing. Colorado has many great riding areas. But, in the green rock parking area(snowies) I talked to some folks from Colorado. They said it is the same drive(time) to the snowies, and they never ride colorado anymore. There will be several colorado plates in the green rock parking lot any given weekend.
Must say something bout the snowies huh?