The TH900 pads are indeed more room. Do they make the sound less focused, or reduce bass or affect stage or stuff in any way? Always been curious. Will need a replacement when my stocks die.
The Hm5 leather angled ones are a downgrade IMO.
I have TH900, Alpha and Omni Pads. In my experience:
TH900 pads - thinner sounding, more airy and wider soundstage I think, a bit prone to sibilance, but bass has more low end rumble. More v-shaped but still sounds pretty good overall.
Alpha pads - YMMV, but I don't like the sound with this. It was an old pads I used with my K551 that I enjoy a lot, however sounds kinda rough with TH-X00 for some reason.
Omni pads - tamed the high end a bit, I think a slightly bigger soundstage and smoother sound overall. Sounds not much different than the default pads, but a bit more refined. Won't suit those looking for treble I think.
Of course, again YMMV