The way I see it is that iDevice has more teams that have a shot at winning each time, around 5 per time estimate. However, the top tier teams that are in mixed, even though there are less of them, are much better. Just going to list guild names to help justify this.
iDevice Top Contenders
VIS
Rebellious
Predation
CRZ
Silver
DOT
Any of those teams could have taken the win in my opinion, possibly more, they just had to play better in game. Mixed, by comparison..
Alumni
Blackout
Apostles
SL (maybe, and this spot would have gone to GTEAM in past seasons, before their team got worse)
So Mixed has less teams that have a shot at the belt based on this. However, if I could take Apostles mixed, for example, and put it in the iDevice tournament, I have full confidence they'd win it, or take 2nd at the very least. Same with any of the other mixed teams listed (except Alumni which would win handily no matter which tournament, there's your bias GOAT).
So if competitive is defined as amount of teams that could realistically win, iDevice. If competitive is defined as the most skill among the top teams alone, then Mixed is more competitive.