I won't help you to decide between the two but I own a pair of X3 together with D2 and this combination rocks. But you won't make a mistake with M5 either.

MylarOne X3 sounds really good - I was impressed at first. But I had to find some good earphone tips. I tried my gf's Sony EX81 tips and they worked perfectly but I don't know where to buy them so I ordered some UE tips. Hopefully they will git better than the stock ones. The stock ones cause my...