I currently have not played either bit this is my undertsanding...

1. The 915H is designed to be more forgiving and have superior distance. It has a very large sweet spot and is a low spin hybrid

2. The 816H1 is even longer than the 915H but does not offer the same forgiveness. It is more of a players hybrid and plays more like a iron than a fairway wood. It will also give you more feedback and a greater feel of the ball with its super thin face.

Besides the color change to a gray from black, the hosel adjustments are in 1 degree increments instead of 0.75 degrees.  Titleist is moving the hybrids to the same release cycle as irons as they are generally included in an iron fitting as 3/4 iron replacements. Both the 915H and 816 1 are intended for the average market.  The 816 2 corresponds with the better player 915 Hd.  The slot technology is continued.  The slot was more than incremental in the 915 from the 913.  The 816 looks to be more of a refinement upgrade.  If like past years, the 915s will shortly only be available as stock on hand in retail sites.

I owed2 915 h earlier this year and sold them. I hit the 913's better. I just hit the 816 H1 and H2 and find them more to my liking. Much easier to hit high and straight. I'm going to get the H1 in a 19 and the H2 in a 23