Connectivity, user interface, and the 2 head guys on the Helix team are both bassists who seem to want to ensure us low enders are properly catered for.
Twice as many amp models on the Helix at the moment too.
Headrush has a touchscreen which IMHO is a shite idea for a floor unit that could be brushed against by a near stage punter. Interestingly the prototype Helix had one too - and they dropped it after testing.
There are endless Headrush v Helix threads on TGP and other places.
Overall, though all the modern modellers are far beyond the old fizztech efforts. Let your ears decide - and your fingers in actually arranging sounds.