I feel well-qualified to answer this one as I have both an Axe FX II XL+ and an Axe FX III. I upgraded because I wanted the latest, but when it came down to it, I feel happier having a "spare" Axe FX than I would having the money sitting in the bank. Not a luxury everyone can afford, so I do feel priveleged to be in this position.
What does the Axe FX III give you that the II doesn't? The most obvious is the improvement in front panel user interface. It immediately felt more user friendly, and the features and adjustments more accessible. (Just as well, because we waited a little while for Axe Edit implementation for the Axe FX III
)
The big one for me was the availability of the FC-12, which we actually waited a while for in Europe, so continued to use my old FX II with MFC-101 until that was available. Sounds like it won't be for your setup, but the FC-12 opens up a world of amp and effects switching possibilities that the MFC-101 couldn't get close to. The FX III has rather more switching possibilities than the FX II as well.
Then there's the sound and feel. The final firmware version for the Axe FX II was a really good one, and I felt like we waited a while for the Axe FX III to be appreciably better. It really is now, but if you don't buy or try and FX III, you won't notice. The Axe FX II is absolutely outstanding by any other comparison, with amp sounds that most anyone could live with. OK, fewer directly modelled effects pedals and amps implemented, but I could make it find a way of doing everything I really needed with the final Ares v2.0 firmware.
So to address
@Sundaday's original question, it depends... The Axe FX II is capable of giving you a lot of very professional sounds, studio quality effects, and because the firmware is essentially frozen, the only "rabbit hole" you will really have access to is 3rd party speaker cab IR files. The extra $1300 for a used Axe FX III will buy you easier editing, more authentic feeling and sounding amps, more effects, switching and channel options. It also buys you the option of regular firmware updates, all the latest modelling technology as Fractal release it. Of course you also then get the opportunity to come onto this forum and argue about whether the new version sounds better or worse, and to make unrealistic demands for new amp and effects models, and ever more complex switching functionality.
@Sundaday, my suggestion would be to get a used Axe FX II so you can get a toe in the water, and get a feel for whether you need the additional features and "authenticity" of tone and feel. You might be surprised, but I'd suggest you'll want a year with it before you get to know how to use it inside out, and really understand any limitations it might have for your own use. By that point you'll have a much better idea of which features of the Axe FX III might be of genuine additional benefit to you, and you can trade up if you feel that way.
Liam