Hmmmm Very difficult question actually...
I like them both equally I guess...
K'nex I had first, and I've got a ton of it, including motors, sensors, lights, control boxes, track sections, bendy bits, panels, gears and pulleys... I've built LOADS of different things with it, admittedly mainly fairground rides, including a 4ft Ferris wheel, the Rippin Rocket Coaster, and even a kind of samurai thingy, which didn't work lol.
I got the cyber K'nex set too which was cool, except it didn't really work properly lol, but I adapted the kit and made new things which was awesome!
Lego? I love that too, mainly because of all the theme things they do, like pirates and indiana jones (temple escape ftw) oh and then there's the added bonus of I get 50% off ANYTHING from legoshop.com and I've spammed that so much lately hahaha.
I used to build movie sets out of lego, unfortunately it's long since been taped over, but one of my best films I ever made was called "pizza dude" and all the sets/props/characters were made out of lego. Good times...