I've heard RMC piezos are a tad better than Ghost, but that might just be down to user preference.
Axon units come up on eBay all the time. Kinda pricey though, and I have seen from time to time non-functioning Axons being sold "as-is/for parts", so I've wondered about their reliability.
The discontinued Yamaha G50 uses the same technology/software as the first version of the Axon units. As I recall, Yamaha basically bought their design from Axon and then manufactured their own hardware. I've seen tracking speed tests done by guitarists that put the G50 with a hexaphonic magnetic pickup (Roland GK or Yamaha G1D/B1D) at the top of the heap - faster than Roland units with piezo systems, and actually a millisecond or two faster than Axon/piezo systems.
The G50 never made it to version 2 so there was never a firmware update to enable use with piezos. Have to use a hex mag pickup. First version Axon units eventually had an update available that enabled piezo use. A lot of players like the cleaner look of piezos over hex mags, so they eliminate the G50 from the start.
I don't have any direct experience with Roland converters, but I know they've never been as fast as Axon/Yamaha stuff. A lot of guitarists actually prefer some of Rolands older, slightly slower stuff because they say it's more predictable - they know what causes glitches and can work around it better. They say the newer Roland stuff is faster, but harder to get good, glitch-free results from.
Never seen a non-functioning G50 for sale, and Yamaha QC and reliability in general has always been very high.
I've been using a G50 with a Roland GK2B for a little while. It works well, but you do have to make adjustments. Latency, even down to low B, is acceptable. Not too hard to compensate by playing on the leading edge of the beat. You have to play as cleanly as possible if you're using a patch that has a fast attack. What works best is to use a mix of your regular bass signal with a synth patch that's set up with a slower attack. This way all the little squeaks and clicks don't get "translated" into wacky sounds jumping out of your synth - since they're over before the envelope of the synth patch is ramping up, the little clicks and squeaks are basically filtered out. Since your straight bass signal is there for the attack of the note, you have no perceived latency.
Setup is critical with these systems, and you have to tweak some things differently for different sound modules. I've found my E-mu Proteus works very well with little fuss, but my Nord Rack2 requires setting the pitch bend range in the G50 very low or to -0-. It's very sensitive to the slightest bend of a string.
I haven't set up this way yet, but if you really want to go straight synth only, I would consider setting up a dedicated MIDI only bass controller. Use something like .030 gauge flat wounds for all strings on the bass, tuned up to a B or C, and transpose the pitch for each string "in the box".
I've heard of using piccolo tuning for a MIDI controller as well, but I figure if you're going to transpose anyway, may as well minimize latency and have it consistent across all strings.
I'm probably going to get a Yamaha B1D pickup off eBay to try out to see if it's any better with the G50. I figure they were built specifically for each other, so it very well could be better than the Roland. Been using the Roland just because I already had it. I got the G50 at a ridiculously low price off eBay.
True, the current "ultimate" bass pitch-to-MIDI setup would be an Axon AX100 MkII with RMC piezo system, but the total price for that could easily top $1000. For the price difference and if you're just trying this out, I'd recommend a Yamaha G50/B1D to see if you can live with the limitations of pitch-to-MIDI.
And ofcourse that's exactly why I don't like to pay for internal sounds and modelling in a GR-55, but rather have a straight converter unit without the extra features.
