Here's my season's line-up as seen from the rear.
On the left is an MT 43 trials tire I ran in the early and late seasons of the last two years, when the pavement was cold. While not exactly a snow tire, the soft rubber of the MT 43 provides good grip even in the cold and allows me to squeeze in more riding up to the limit of me staying warm. The penalty is fast wear and poor mud performance. Haven't looked at my maintenance log, but I'd be surprised if I got more than a few thousand kms out of it. Already a square design, it squares off even more on pavement and should really only be used off road on hard-pack surfaces and at low psi. I can't see myself choosing this tire again for DS riding.
Next is the K760 mounted on the bike. Often available for around CAD$90 or less, it's one of the cheaper options for the rear and provides great performance for the price. This is what I rode in the Roaming Rally in 2015, pretty much using up 80% of the tire's life in 1000km of aggressive dirt riding completed in 26 hours with a loaded bike. As a relative novice in dirt, I found this tire provided great traction and control, and allowed me to corner at speed with increasing confidence because I could feel when it started to break away and then use throttle to control the turn. On pavement it also performed well, giving predictable cornering and braking, and wearing reasonably well considering it's a full knobby and most of my riding was done with luggage. I'd probably run one of these again, given the great value it provides.
Third is the shiny new D606. It's similar in design to the K760 except the knobs are bigger and not subdivided. The D606 is slightly narrower across the knobs although the carcass width is almost identical.
Here you can see more clearly how the MT 43 and K760 have worn down. Nice new, rounded profile on the D606!
Here's the MT 21 (right) compared to the Scorpion Pro. Much bigger knobs on the MT 21. The MT 21 is slightly narrower than the Scorpion across the knobs, but the carcass width is almost identical.
Although initially happy with the Scorpion Pro, I never really became comfortable with it and thought it was just my riding ability that needed to improve. On gravel roads I found the tire wandered unpredictably, despite playing with air pressure from around 12-20 psi and changing riding position. At low pressures it provided little impact resistance and I managed to ding my rim. Running around 18 psi seemed to provide the best compromise between protection, handling, and traction, although it would still squirm and never really felt planted. The bars would sometimes oscillate but fortunately that never led to a tank-slapper. Haven't had this with any other front tire. In the end I concluded that yep, the tire was a significant part of the equation for my type of riding, and vowed to try something else even though there's still some life left in the Scorpion.
Mounting the D606 and MT 21 was straightforward after leaving both wheels and tires in the hot sun for half an hour. They both spooned on easily with just a little talcum powder for lube. The rear was mounted with one rim lock.
Test-riding the D606 and MT 21 on gravel roads and a rough, rocky section of trail immediately revealed an improved change in character for the bike: there was a much more planted feel. I think the bigger knobs are the most significant factor here because they allow for less squirm. The front didn't show any wandering compared to the Scorpion, and overall traction and control was excellent on loose rocky terrain.
In thick clay mud on the Mohr Road (see below--a nice little track near Ottawa!), both tires also performed well, although lateral traction (i.e. sliding sideways into ruts) was reduced on the rear in particular--as expected--given the more rounded profiles of both tires and the less open knob pattern. However, given this range of terrain it's difficult to find one tire that performs well in all cases. You have to pick your operating point. For me, clay mud is sometimes unavoidable but mercifully short, and my bike is light enough to haul out by hand. So the mud performance of D606 and MT 21 is fine.