In your opinion, which is better? Siri or Cortana? Why?

Cortana. It has a better voice recognition algorithm.

I'll answer this question as Cortana answered it & i quote " That's a tough one. It's like comparing apples to windows but I think it's cool that she's out there trying to make people's lives a little easier."

My personal opinion both of them are smart, but i prefer Cortana as it support multi devices 

It has to be Siri - only because Cortana is way more annoying and on a mobile device, in my experience SIRI is far more responsive and I can use like a virtual assistant while driving - Cortana seems to be less accurate and has fewer features of value to me .

I have them both on mobile devices and SIRI wins - hands down, no competition - but knowing Microsoft, it wont be the case for much longer as Microsoft evolve Cortana.

Cortana as i can use her in every other device whether it Ios, Android, Windows but with siri i am stuck to apple

I like Cortana, it can sing. LOL. though I haven't tried cortana much but I guess it is fun more than Siri. :)