A: Indeed, Seven Dragons is mostly a “re-skin” of Aquarius, but, just like when we make new Fluxx versions, we did try to change things up a bit to add something new. For example, Seven Dragons has a designated starting card, the Silver Dragon, which changes color throughout the game based on what Actions are played. Aquarius also has an extra Action. Those are the the main differences.
We do have a promo card which you can add to Aquarius which acts like the Silver Dragon, if you want to retro-fit that effect into your Aquarius game. If you do add that to Aquarius, there’s that extra Action which happens to change the Silver Dragon back to Wild, which is how it starts. So, if you want to add THAT capability to your Seven Dragons deck, we have a promo card for that (though, to make them identical, technically you should add THREE of them. Whoah.)
Other than that, Aquarius includes diagonally split panel cards, but no 3-panel cards, and Seven Dragons has 3-panel cards, but no diagonals. I’d consider that pretty inconsequential to gameplay, however. It’s mostly stylistic. The dragon art looked terrible when divided diagonally, so Andy came up with the idea to include some 3-panel cards.
You probably don’t need to own both, unless you’re a completist, or have different groups you want to play with who like one or the other.
Decide which art you like best and get that, and if you want, add the applicable promo cards.