There are some key differences of course. The Volca series is made up of different synths for different jobs (drums, keyboards etc.), and each model costs around $150. Pioneer's AS-1 is a straight up sound machine (though it will sequence those sounds into "songs"), and it weighs in at $499. The AS-1 is also slightly bigger than it first looks. The preset sounds it comes with cover all the bases -- from strings to crunchy "acid" noises, but the real fun will be making your own.
While the AS-1 is a Pioneer product, it's actually a collaboration with Dave Smith Instruments (based on the Prophet-6 if you're wondering). This co-branding adds a chunk of credibility to Pioneer's first venture into sound making, and somewhat explains the higher price.
The monophonic AS-1 will also play nice with Pioneer's other gear, and not just the original Toraiz. If you own a DJM mixer and CDJs, you'll be able to hook them all up together and "play" the tracks from the AS-1 via the CD turntables. That is, once it lands in stores some time in March.