I'm afraid I only have the 2 in my sig for comparison, but I think what it comes down to is how much diffusion do you want? For hardly any go with the ShowerCap. For something in the middle get the HydraTree/ShowDome. For maximum diffusion, the Swagger double chamber. Which HT do you think you will use the most? Its going to depend on your personal prefernce for taste/diffusion/smoothness etc . . .
If you are clumsy I would avoid the HydraLine
There is also the straight vs bent neck to consider. I prefer straight since I medicate standing up. I also prefer the feel of a "tube" vs the smaller "trumpet" style mouthpiece.