Before I comment on this, let me say that one thing I like about the Thoth Tarot and ideas behind it is the whole Kingmaking story... the idea that the Prince becomes the King, and the Princess becomes the Queen, and the Courts are wrapped up in the Cycle of Kingmaking. It helped me immensely in beginning to understand the Courts and their place in Tarot.
As far as the Princess vs. the Queen of Cups, I don't see them as that similar, honestly. They both have the emotional tie to Cups, but for me, they hold very different 'purpose' within them (though of course, they are essentially the same person at different points in the Cycle).
The Queen is water of water, motionless and eternal, everywhere and nowhere, all feelings and no feelings. Like the ocean, she appears calm at her great depths, yet is really a sea of every possible emotion, every drop of water. I especially follow the notion that she reflects back everything that is within her sphere. The BoT says, "Everything that passes through her is refracted and distorted. But, speaking generally, her characteristics depend mostly upon the influences which affect her....It may really be said that, normally, people of this type have no character at all of their own, unless it can be called a characteristic to be at the disposition of every impact or impression."
That is the doubling of water, making her seem simply a mirror of everything around her. Is she anything at all? Of course, she is, but it is impossible to pinpoint it and grasp it, just as you cannot hold water in your hands. She is, truly, what you want to see or don't want to see. Her Kingdom is that of ebbing and rising emotions, yet she appears to rule still waters. Still, in the picture, because what is reflected is peaceful. However, if met with fury, would the waters boil? Perhaps, I think.
She is, of course, the purest Queen, and the pure essence of what a Queen is in the King-Making Cycle. She is receptivity and reflection. She creates, but she is not creation. The Prince kills The King and wins The Princess, who becomes The Queen. Thus, The Princess only becomes The Queen because of something that happens 'to' her, which correlates with how we believe feelings to appear. Controlling ones emotions is not about actually being able to create or destroy the emotion itself; it is about something different. So, the Queen, with every emotion at her disposal, becomes that perfect control of imperfect emotion.
The Princess of Cups is earth of water, much less pure. She is trying to adapt emotions into something tangible. So, she is in motion, because she cannot balance perfectly still, as The Queen can. She is the beginning, but she is also the goal. I like that she represents 'crystallization' because I've always thought she represents the idea of emotion (water/cups) becoming real/tangible (earth).
The Queen is about possibilities, she is infinite water, she is everything received and reflected, perhaps distorted or refracted, but kept in balance. But The Princess is about making just one of those possibilities real, as you cannot make infinite possibilities real (they'd counteract each other and become 0).
So, The Princess is less, and that allows her to exist in the tangible, real world. Yet she doesn't access the full power of her suit, and as such, doesn't achieve the emotional maturity of The Queen. The water sways her, she is off balance, she must keep moving to retain her grace, to make things real. The BoT mentions her dreamy qualities, however, it also mentions, "On a superficial examination she might be thought selfish and indolent, but this is a quite false impression; silently and effortlessly she goes about her work." The word 'work' strikes me as significant there, because it associates with her 'earth' qualities. She is the work of emotions.
The Queen is the infinite manifestation of that work. She is done, but it is not as simple as 'matured' because in being the end, she is also the beginning. At any rate, she is stable and all that comes with that---including being stuck, being in one place, as The Queen would be. Her course is set. She is her Kingdom. She stays at home.
The Princess of Cups is a bit muddy, though. She cannot do much on her own in the way of manifesting emotion into something tangible, so she often becomes more of an 'influence' than a product. And the BoT also mentions, "Rarely, at the best, are they of individual importance. As helpmeets, they are unsurpassed."
This makes sense when you consider her evolution to The Queen, who is really nothing at all (and yet everything) on her own. Neither of these characters are much good individually, and they both act more as 'influence' than anything else.
Anyway, just a few thoughts. I love the Thoth Courts.
