I think that mstateglfr makes a good point about comparing rim-vs.-disc weights at the same price point, otherwise the arguments turn vague.
Speaking of price points:
At the really lower end, where flimsy suspension forks might still be used with either brake type, the tendency for the steering to pull sharply to the left when a front disc brake is applied can be enough to steer a sudden-braking rider right off of the trail!
This explains why a disc-brake fork needs so much added flex resistance built in, with it's own effect on a bike's compliance.
The same fatter tires that tended to encourage use of disc brakes help negate the added fork stiffness, but you'll need MUCH plumper tires to exceed the overall compliance of a similar rim-braked bike if the build priorities, weight and and price point are kept about the same.
...But the weight
won't be the same at the same price point, not until there are no rim-braked bikes left on the market with which to compare!
I marvel at how good the V-brakes on my $40 (used)
Schwinn department-store hybrid are. With little setup fussing, and with no tendency toward twisting the suspension fork having lowers built entirely from steel sheet (ok, the dropouts actually appear to be plugged-in alloy forgings).
I added much fatter 2.9x2.1" tires, and the V-brakes had no issues with clearance. Yeah, kind of needed the single front brake booster to realize GREAT braking because of the flimsy 24mm-diameter fork!
Loving the feel of the Hutchinson Python tires by the way, both on pavement and on dirt.
