Passive is totally fine on a 600, and I'd still go for a larger exhaust. Again with the need to control temps, ya know?
The local grow shop here was putting together a package that includes a tent, exhaust, 600digi complete, climate controller (oooh! trickness!), carbon scrubber, and ducting for only like $1200 (I'd have to check but it is a ridiculously low number) TOTAL, whcih is a KILLER deal on all those toys. The shopkeeper is someone whose instincts I trust, he encourages use of the 600digi in the space, and one of my buddies just picked up that pile o' goodies and is currently veggin' away like a mofo.
Oh, and check out the odorsok carbon filter... it's COOL. (and cheap and easy to get rid of and even machine washable! hahahaha)

I'd have to be a sadistic bitch to tell you to go for the K.

The jury's still out on that one though...