I've ridden both of these bikes and several other comparable models. I now own the Kona 153 29. Like any other good bike advice, I'd say it depends on how and where you ride more than anything.
With that being said, I preferred the Kona because I like riding pretty chunky stuff and having the forgiveness for some poor choices. The Kona is all about fun. I do think that the Stumpy Evo was a better pedaling bike, but the Kona isn't bad at all. The Kona was a far more capable descender especially on the steeps. I do remember how light on it's feet the Stumpy felt...
My advice would be to ride them both if you can, preferably on similar terrain to where you plan to ride. If you want a really fun bike for gnarly terrain, I'd go Kona. If you want an efficient yet overly capable trail bike, I'd go Stumpy.