I watched the Netflix film first, and then thanks to the reviews on that one, discovered this Danish original and watched that too.
The Netflix film is a taught and effective thriller, with a great central performance from Jake Gyllenhaal. It's mostly word-for-word the same as the Danish version. And it's free if you have Netflix!
The Danish version is £2.99 on Amazon. It's less stylish and less melodramatic, but much more believable and affecting. I cared more for all the characters, and despite knowing exactly how it was going to play out, I was hooked throughout. At times I couldn't look away.
The Netflix film adds an unneccesary personal arc, some wildfire-related melodrama, and a set more like a James Bond MI5 office than a 911 (or 112) call centre - all of which reduced the impact and made the film less engaging.
If you're definitely not going to watch the Danish version, the Netflix one is a good bet. If you're ok to watch either, choose this original - it's a lot more suspensful and completely captivating. (Ideally, unlike me, don't watch both!)