AS-1 is a single voice of the Prophet-6. It's extremely capable of performing simple bass lines. The Pro 3 has a much wider sound palette and many more advanced features.
If you just using it primarily for bass line duty and budget is a consideration, the AS-1 is a great solution. It does a lot of other sounds too, naturally. It's easy to program and it's powerful. If you want more complex sequencing, modulation, and sound capabilities with more hands-on control, the Pro 3 is worth looking into.