I prefer the former, which is how it's set up right now. By having it be an enhancement bonus, it doesn't stack with gauntlets, belts, and Bull's Strength.
I regret posting the version I did so hastily. I think I should probably nix all of the save bonuses. Perhaps I could keep a few of the Reflex saves, such as the aerial form's save. I'm also fine with toning down the Str and NA bonuses a bit. Again, I copied pretty close to the PHB2 version, which wasn't factoring magical items in.
Earlier forms, I think are fine as-is. I don't see much reason to alter the predator form or it's aquatic counter part.
Aerial form is pretty good as-is, which is really just used for flight anyway.
I could tone down the bonus on the two slayer forms. By that point, you're assumed to have some pretty decent gear anyway.
The avenger/horror forms are really used to gain your first good reach. That hit to mobility will hurt, but this could easily be augmented with magical gear. This one might need to be toned down.
The elemental fury form saw the biggest change from the PHB2 version. Originally, the only difference between them was the eneregy resistance you got. Now, the air elemental gives unparalleled mobility, and the earth form gives a different type of mobility (almost like blinking or short range teleportation). I'm not sure if handing out the elemental abilities was a good idea or not. At 18th level, I don't think they're overpowered, but this is a full-casting class too.
Midnight, how are you thinking about running testing? Is this a simple test of levels 1, 5, 10, 15, and 20? How are we going to pick challenges? I can do my best to guess what shapeshift should and shouldn't include, but it will be tough to hone it in. Should we work on spells a bit too? That could have a big impact on this class.