A number of forum members have tried Line6's Helix and/or Fractal's AX8, and have written about their experiences. We've not had any review of either unit. This might be because both units are much more complex than a guitar or pedal, So this thread is for collating what others have said.
For those people who don't have the patience to read everything the conclusions are:
1. Both units are incredibly capable. Never have guitarists been so well served.
2. There is no "better". It all depends on what you want to do with it, and how happy you are to learn how to get the best out if it.