The two side seats and the waterfall seat are the deeper seats in the spa with the remaining ones being shallower. I will note that I don't personally consider any seat in the Phantom to be a real deep seat though. Being that I don't have the Niagara on my floor, instead I have the Journey which is the same except without the vanishing edge, I am not sure of the price difference off hand.
I must admit that if you were a tall guy then chances are none of these negative edge spas would appeal to you, but that's the beauty of it, there always a spa out there to suit everyone. For me and my situation the Niagara is the ideal tub.
P.S.
On a different point, why are Coast the only ones adopting the commerical pressurized filtration system and what's it's real benefit over the rest?