I Ride a 2006 DRZ400S off-road for a living. Road legal knobblies are always going to be a compromise but The MT21 is a fantastic tyre on and off road and will only clog up in very clay-like mud but will quickly clear out once on slightly harder pack dirt. On the road i have full confidence in scraping the pegs around roundabouts and they are pretty decent in the wet (but a little naughty when brand new). I reckon i get about 2.5 - 3 thousand out of each tyre too, and i don't let them get too badly worn down so you'd probably get more if you were prepared to take them down to the lower limits.
I have had experience of TKC's but found them very twitchy in the wet to the point of spinning the back wheel on some poorly surfaced B roads around the dales.
For trail riding go with the MT's in my opinion, but for anything else (enduro etc...) Carry on looking for something more aggressive