Wow, this is a super hard dilemma! I'm not sure what I would do in this instance. This isn't quite the same thing, but a couple of times in the past, I have traded dearly-loved pieces (ones that were among my favorites for a series or character) for things that were wish list items. In the end, I was lucky enough to buy back the stuff I had traded, although I suppose the fact that I bought them back shows I shouldn't have traded them, in the first place.
It sounds really hard to compare the two pieces objectively. Defining scene vs. true beauty is difficult, as both are equally rare and special -- but for different reasons.
Art is very emotional. Our connections to particular shows and characters are very emotional, too. They come from the gut and the heart. So my advice (similar to what has already been said) is that you should think about your emotional attachment to each piece. For the one you have (defining scene) , when you think about it no longer being in your collection ... how does that make you feel? Do you feel that twisting anxiety in the pit of your stomach that tells you you won't be able to let go of it, no matter the price or offer? I've been surprised, in the past, to feel this with pieces I wouldn't have expected to. For the piece you may trade for (real beauty), how do you feel about it being in your collection? Do you feel it was "meant to be"? Do you feel you would treasure it as much or more than what you already have?
Either way you go, it sounds like you won't make a mistake. Because, either way, you will have a wonderfully special and lovely piece of art in your possession. I say trust yourself, take a deep breath, and choose from the heart.
(I know ... this was probably no help at all.
)
