Zedd's Clarity peaks:
Australia 13 (Gold - 35,000)
Belgium 69 Canada 17 (Platinum - 80,000)
Germany 94
Ireland 39 Netherlands 81
New Zealand 16
Scotland 23
Slovakia 66
UK Singles 29 US Billboard Hot 100 8 (Platinum - 1,200,000)
US Pop Songs 2
US Adult Pop Songs 20
Paramore's Still Into You Peaks: Australia 5 (2x Platinum - 140,000) Belgium 60
Brazil 39
Canada 78 France 189
Ireland 6 (Gold - 7,500)
New Zealand 14 (Gold - 7,5000) Scotland 13
Portugal 25
Poland 26
Philippines 1 UK Singles 15
US Billboard Hot 100 36 (500,000 so far)
US Pop Songs 21
US Adult Pop Songs 22
As you can see both singles charted in almost the same countries with Clarity charting in Germany and Netherlands where Still Into You didn't chart, but the latter one charted in the Philippines and Brazil.
So if Stay The Night smashes? is it because of Zedd or because of Hayley Williams of Paramore? or 50/50?
I highlighted the song with the highest peak in each country.
"Clarity" is probably the main reason why it's doing so well however, so I'd say it's more 60/40, in Zedd's favour. But it's still both, so I voted for the third option. In most cases, people aren't here for solo artists outside of their groups. Hayley Williams is The Only Exception to this.
I'd say both. It's obviously seeing success based on "Clarity", but Hayley really delivered an amazing vocal. It seems like people are here for her features