I had already email you, but I will repost here for others with the same question:
It depends on how
you are going to use the product. The S4 is optimized for compressed audio –
typical mp3 size files, while the Custom 2 has a more flat response, which takes
better advantage of “lossless” audio (not compressed). They also fit
differently, with the Custom 2 being over the ear and a bit more challenging to
achieve a proper seal (for best bass response). The S4 is more simple to
use.
I hope that
helps!