They both had similar chart runs:

US:
Dark Horse: #1 for 4 weeks, 22 weeks in the Top 10, 2 weeks & #1 on Radio Songs, 5 weeks @ #1 on Digital.
E.T.: #1 for 5 weeks, 20 weeks in the Top 10, 5 weeks @ #1 on Radio Songs, 7 weeks @ #1 on Digital, and hasn't it still sold more?

WW
Dark Horse: #2 for 6 weeks, 20 weeks in the Top 10, 39 weeks on chart
E.T.: #1 for 2 weeks, 15 weeks in the Top 10, 26 weeks on chart

imo I think E.T. is, and will be more remembered. DO NOT count streaming into this, because we all know if YouTube was that big in 2011 E.T. would have 500m~ views by now. Back then, I heard E.T. on the radio far more than I heard DH, and people at school were talking about the "alien sex" parts and just the alien thing. Also she sings it better live. Dark Horse has the weeks on chart tho, and the performances.

So basically,

Dark Horse

In the US: ET has sold ~5.7 million copies in 3+ years; DH has sold ~5.6 million copies in a year.

E.T.

Dark Horse is an absolute monster I'm going with it.

The horse.

E.T was bigger on the WW chart? Shows how reliable that is then

Dark Horse. E.T. just seems like a flash in the pan in comparison. Dark Horse is still HUGE a year after it was released as a promo single.

As big as E.T. was, I think it sort of ended up lost in the other monster singles that made up the Teenage Dream era. When Dark Horse smashed, it was pretty much the only thing you heard from Katy, even dipping into when other singles were released. For that alone, I think DH will be more remembered.

Dark Horse was even bigger than Firework tbqh

.

Also the most popular track WW. I love both tho.

Dark Horse was a pop phenomenon.

Dark Horse is arguably Katy's biggest hit and one of the biggest pop girl hits of all. Only Roar can rival it.

Dark Horse.

Damn

Isn't Firework bigger than Roar?

Tbh no.

A horse is a horse of course of course no one is bigger than a horse of course.

This. Chart-runs, peaks and sales aren't everything. Firework will forever remain her biggest hit.

E.T >>>>>>>>>>>>>>>>>>>>>>> DH