Some folks find Hoya filters to be difficult to clean without streaking, so the HD coating are better for their streak resistance. On the other hand, they are very slightly less transmissive (99.35%) compared to the S-HMC (99.7%).
The HD is pretty much the same as the HD2. They came up with the HD2 to help control the trade of gray market merchandise. If they're selling the HD filters for a decent price point, that's what I would go for.