If you're happy with the sound and features of the QC as it is, its smaller size and existing ecosystem can be pros for a lot of people.
If I had to choose, part of me would want to go Helix Stadium for the eventual better captures (unless NDSP does some sort of update here), for Line 6's excellent support track record (!), for the better UI and screen (by all accounts), and for the presumed better build quality / quality control (and warranty service), but on the other hand the smaller size, multiple instrument inputs (when compared to the non-XL Stadium), and better reverb would all be big real world pluses for me. Helix's reverb is still lacking imo, and some of their modulation is too, and it doesn't seem like that will change soon, and the Stadium's size would involve me really changing around how I set things up, and at the gain of a lot of features I probably would not end up using.