I was reading the info on the FI from Pandamidi. It's a different approach to synthesis than the Roland GK.
With the FI, any bass can connect to it, using a standard audio cable. It's "monophonic" in that it responds to a single note at one time. It references a MIDI input connector for uploading patches; it also says you can connect a MIDI controller to it and play it that way. It appears that the MIDI OUT port is actually MIDI through, meaning it will pass MIDI from an external controller to another synth, but won't transmit it's internal MIDI data (it's not clear to me exactly how they do it).
With the A4 SA and a GK interface, you need a special GK cable. Each individual string sends it's analog audio information to the GK (or in my case, the Roland GI-20), where it's converted to MIDI. This is a "polyphonic" solution, allowing up to 6 (for a 6 string guitar) unique tones to be played simultaneously. And, you can play multiple notes of the same patch (chords). That MIDI data can be transmitted to any MIDI device such as a keyboard, or a computer interface, or ?
IMO, here's no real comparison between these 2 systems, although they might be used for the same purpose. If you are looking for simplicity, at an affordable price, then the FI is the best choice. If you want to really get into MIDI, and the expense/complexity doesn't deter you, use the GK-based system