I've seen both. I loved the Killing because Sarah Lund is such a brilliant character and if you compare her directly with Laure in Spiral she is mesmeric - so focused and intelligent, whereas Laure is more needy and shouty (although not always). And it was such a long series that it felt like a big emotional investment.
However, Spiral is in its third series and it's got a great ensemble cast - it's not just about Laure but also Roban, Josephine, Pierre, Gilou, Tintin, etc. I thought the second series was just as gripping as The Killing and I'm finding the third series got into its stride about halfway through in terms of tension, plot and dialogue. Also, it's so much more French than The Killing was Danish and has a great sense of place - all the street scenes, French interiors, etc. For Francophiles or fans of French film in general it's hugely enjoyable.
I'm slightly nervous about series 2 of The Killing being overhyped. I think I'll also miss Troels, Rie, Meyer, etc. Still can't wait to see it, though.