There are any number of directions you can go with this. Assuming you want the ability to access both traditional bass sounds and synth sounds, there are several different routes you can go. For the traditional route, you can obtain synth sounds via pedals or triggering sounds via a GK3B PUP, for example, (or the aforementioned Industrial Radio bass) or you can go with a non-traditional instrument like a Ztar Bass, Misa Digital, even a cheap midi controller like a You Rock guitar which have strings to pluck but fretboard pads instead of frets but with zero to little latency and then try to "recapture" real bass sounds via good software. I've tried nearly all options, and they all have their advantages and disadvantages, so it really comes down to how deep you want to get with all of this and what your needs are.
When all was said and done, I wanted a setup that had:
1. Synth sounds obtainable via my conventional bass setup, so I can switch between the two sounds or even blend both
2. LOW latency on all strings. This is key for me, as none of the pitch-to-midi systems out there can track bass frequencies quick enough for pitches lower than A. I don't want to be limited to playing high up on the neck just to obtain synth sounds
3. Cheap setup. That rules the Industrial Radio Bass out for me, which will now run you over $4000 with shipping.
4. Unlimited access to sounds. That rules out the Akai Deep Impact and other pedals for me, which have limited sounds
5. Minimal glitches. If you're in the pitch-to-midi domain, it's almost unavoidable, so it comes down to how well you can avoid false triggers or concealing them
After much time invested in experimenting with different setups, the best setup I arrived at for both recording and live was my Carvin L50 strung with piccolo strings, my Roland GR-33, and a Digitech Whammy. The piccolo strings give me the ability to play any string and retain good low latency, while the Whammy serves as my ability to capture "real" bass tones (simulated via octave down setting) when needed. (Jack White uses the Whammy the same way for replicating bass). The beauty of the Carvin LB50 is that it has stereo outs, so I can actually run it out to a guitar amp, bass amp, and PA system simultaneously, using the piccolo feed to reproduce guitar sounds, the Whammy feed/octave down to my bass amp for bass, and the GR-33 out to the PA for synths. Instant 1 man band!