I think Fernando S isn't taking into account ZiNc's dynarec core vs MAME's interpeter core. This of course takes tons of horse power to get full emulation (60 fps) with full sound. If MAME at some point in time gets a PSX dynarec core then the games supported will sound nearly the same in either emulator. Let's not forget MAME's new Namco System12 sound comes from the ZiNc authors :-)
Well, assuming your system runs the games 60/60 on both, the SPU in MAME 0.84 sounds pretty much just as good as Zinc's. (Zinc has reverb emulation, which Strider 2 piles on, but generally ZN games don't use it much).
I meant that in Mame 084u2 in my computer the sound isn't as good as it is in Zinc. I think it is because my computer lacks processing power. But I must admit that the sound in the namco12 driver was a major achievement.