Sorrodje, your post was extremely helpful, even though I couldn't do a Google Translate yet, as I'm on lunch at work.
Overall I think I'm drawn more to the sound on the T5P's (from everything I've read), than to the Sig Pros. It's just that the Sig Pros are so portable...they'd be perfect for me if they kept the look and had the T5P sound. Another consideration of mine is that I like to fall asleep with headphones on. This has never been a problem with the QC3's, as they're quite good at isolating and are very light-weight and relaxing to me. With the T5P's....is sleep even possible with those huge cans on your head? With the Sig Pros, I'd imagine it's quite possible.
In terms of sound quality, I know at this level, there won't be much of a significant difference, just preferential differences, but for Enya, post-rock, and classical (I also like Rattatat, bands like the XX, Bon Iver, and various others) it seems like the T5P's are probably the superior cans. If that is the case, I will forego that bit of portability and convenience and aesthetics and choose the T5P's.
Do the T5P's surround you with sound the same way the Sig Pros are supposed to? And I do apologize for all the questions, I just like knowing what to expect, or at the very least, accumulating enough information to make an informed puchase.