I don't have experience with the W4, but I've heard it has more sparkle than the SM64. The SM64 definitely has great detail, what I would say are good vocals/mids, and a big soundstage, but I wouldn't quite call it neutral and sparkly. I think of the SM64 as a richer, warmer sound in general. The SM64 also has much deeper bass and a wider soundstage with an amp, which is something to consider.
Nope, speaking as someone whose owned the W4 it's the exact opposite actually. The W4 has roll off in the upper registers and sounds highly polite in the treble. It also has less detail, less soundstage and sounds less bassy when compared to the SM64.