I know I picked Snowfall just a few days ago, but I think I'd be tempted to switch over to Halls of Mist if I made the picks now. In theory you could just hold it in your hand, wait for an opponent to make a big attack, then drop this land to shut down those attacking creatures for a single turn. But this card won't have dealt with the initial attack and won't buy more than that one turn. I guess you could use Solemnity or something to get rid of the cumulative upkeep issue, which would then give you a land that slowed creatures down to only being able to attack every other turn. So it's probably still better than Snowfall.