You can do SP on both but man.. they're so fukking different.
I'd go with Horizon because there's no real barrier in that game. You progress really easily.
MHW has a lot of barriers of progression. You can't just put in an hour a day, you have to grind for resources to build better gear in order to progress.
I do SP on MHW and that shyt was a chore. By the end of the game you're a killing machine but you're weak pretty much 80% of the time, it's a false sense of power. New weapons still hit weak but go up maybe 10 hit points.
All that said, Horizon is all about the story. You'll feel powerful as fukk early on and throughout the game.
Get Horizon.