I love how no matter what you ask in this forum, some people will just ignore the question entirely and tell you what they think you should do.
I love how some posters can't create threads properly and then yell at people who make constructive suggestions. Your thread title has "OR: the ultimate synth cabinet" right at the top of the page. What did you expect? That no one would offer suggestions about that part of the subject? Geez.
Second, Greenboy only has 2 cabs which have two 12" drivers: the fEARful 1212sub (two LF drivers only) and the fEARful 1212/6 (two LF drivers, a mid driver, and optional HF horn). Those two designs are for completely different applications. The 1212sub isn't designed to put out midrange, and IIRC has a low-pass filter crossover with a cutoff frequency at about 850 Hz, so that one certainly doesn't seem to be what you're after. I'll assume then you're referring to the 1212/6 as "the Greenboy 2x12".
Avatar's B212 cabinet has nothing in common with the fEARful 1212/6. The Avatar cabinet uses either a Delta-12lfa driver (or a similar OEM model) in "full-range" configuration where the pair is asked to cover everything from the low-end all the way up to the Foster compression horn, likely ~4kHz or so. If you look at the frequency response of the Delta-12lfa driver, it isn't very flat in the upper midrange. So some of your upper midrange is going to be exaggerated, and some of it will likely be missing.
Hence the first advantage of the dedicated 18sound midrange driver in the fEARful 1212/6, which is a more "even" frequency response. The dedicated midrange allows the Kappalite 3012LF to handle a more reasonable range up to ~850Hz or so, while the mid driver either handles ~850Hz on up, or ~850 to about ~4kHz if you use the newer 3-way crossover design with a dedicated HF driver. The result is a more even response across the frequency spectrum, and a more hifi tone as a result. You could send a PA signal through it and it wouldn't sound bad, unlike the Avatar B212.
Plus, the different drivers in the two cabinets have completely different XMax, resonance frequency, etc. They are going to produce different volume levels, yield different response curves (especially towards the lowest end of the spectrum), and have different responsiveness characteristics, etc. I would expect the Kappalite 3012LF in the 1212/6 to win that battle hands-down, and the higher XMax will likely allow it to move much more air, making it significantly louder. It will likely also reach lower, producing frequencies that the B212 can't touch.
Finally, there are other aspects like the weight. The fEARful will be almost certainly be lighter, even if you spec out the B212 with neo speakers. Avatar isn't known for making lightweight cabinets.
Honestly, if you want to compare any of the Avatar cabinets to the fEARful line, it should be the TB153. It has a 3-way design much closer to the fEARful, and there have been entire threads dedicated to discussing its pros/cons. I have a 4-ohm TB153 and love it.