Both suck in my opinion. Remember that the MX400 was originally supposed to have the new Aureal chipset; however Aureal tanked and they picked some unknown company at the last minute to develop a brand-new chipset instead. The older MX300 is MUCH better because of this.
The MX400 was supposed to save diamond, this was probably why they tanked too. Diamond sucked anyway, they sent out ONE driver for the MX300 in a year and a half, and it was a BETA. Their driver support was the worst in the industry. Good thing Aureal kept updating the reference drivers.
I have an Mx300 and love it, much better sound quality than a Live! (which I also have). Note though that A3D is not being supported in any new software. If you still play Halflife/CS/TFC it's invaluable though (especially CS -- EAX is not really a true 3d soundspace, while A3D is, so you can tell exactly where those footsteps are).