Ice Shard will provide better damage output, while Powder Snow will gain energy faster and allow you to use Bulldoze faster.
If your goal with your Piloswine is to get to your charge attacks as quickly as possible, then go for Powder Snow. If Piloswine is also your go-to choice as an ice-type, then you may consider Ice Shard, as it hits harder. Or, you might want to look at unlocking Avalanche as ground and ice moves are a powerful combo in PvP.