I have not written yet to compare them directly, but I have both.
But here's my take on things.
The Pioneer is more balanced, with a more natural bass to me, and is more tolerant with recording that are tipped up in the highs.
The Sony is prone to some searing highs, that's why I use the Sony-Kimber cable with it to deal with it.
I just find the Pioneer more refined, which to me means that it is easier on the ears.
I also love the detail on the Pioneer, where little background things are more developed than on the Sony, and in fact more developed than most headphones.
The bass is prominent in both of these headphones, but (especially after a lot of listening and I think break in with the Pioneer), Pioneer bass is not as pushy as the Sony.
The Pioneer is just plain fun, and not assaultive.
If I am sounding like I prefer the Pioneer, well, the answer is yes.
I don't find the Pioneer heavy at all. I have even thought about or noticed the weight at all. It's a really comfortable headphone in every way. I am kind of obsessed with comfort in my headphones, and the Pioneer is great for me.
I'm keeping the Sony to complement the Pioneer because it's still nice and interesting and unique and I like variety.
But if I had to choose, the SEM1 is my choice. To me, it's a very special headphone.