I'm definitely looking forward to getting the details on both devices, but I know not to be in too big a hurry either. I know I would benefit greatly using either device. Too early to say which one would be the best choice for me.
Pretty much all details of the FX8 have been posted:
http://www.fractalaudio.com/downloads/misc/FX8-Onesheet.pdf
Minimal details on the AX8
It is not an Axe-Fx. It won't be able to do two amps or two cabs. It won't have things like the Vocoder and other esoteric blocks. It's a stripped down floor version at an aggressive price.
It will have G3 modeling and user IR slots. It has two dual-core DSPs, one for amp modeling the other for effects. These DSPs are slower than the ones in the Axe-Fx so it won't be able to do two instance of amps.
There will be block compatibility between the Axe-Fx and the AX-8 meaning you can copy blocks from your Axe-Fx presets into the AX-8.
It has USB, XLR outs for FOH, 1/4 outputs for local monitoring and an FX loop. Four expression pedal inputs. AC powered, no wall-wart.
By the look of it the FX8 is purely the effects from the Axe-FX 2 (not surely all the effects from the Axe-FX will be in it) and the AX8 is just the amp models with some basic effects such as reverb. I'm looking at it as being the AX8 is pretty much Fractals version of the Line 6 POD HD500X and the FX8 is Fractals version of the Line 6 M13.
Obviously the Fractal will be MUCH better than the Line 6 but thats the closest comparisons I can draw from it.
As to which to buy depends on your needs. If you're happy with your current rig and just want more effects then the FX8 is the way to go. If you want an ultra portable rig in the form of a floor board then the AX8.
This is just speculation as not much has been said about either product.
The AX8 will have 2dsp but.... Not all the amp models that has on the axefx XL or mark II, I...