If it gets a buff, it should be much more expensive.
+1 is really no difference at all at the end of the day, be it damage or speed.
7 length is basically nothing, it's not even really noticable on the battlefield, it's what, 0.1 of a second more running before you can hit? If that?
0.5 Weight really makes very little difference, the Nodaichi will stun more often (barely) yes, but it'll also slow down your running acceleration more as well, so it's even there.
That just leaves the cost, which the Dadao is under half of what the Nodaichi is - this doesn't factor into if people will buy it or not, true, but it does factor into repair costs, which is significant. Either make the Nodaichi cheaper, or don't buff the Dadao.
The way I see it, the Dadao is a middle of the line sword for middle of the line use, at which it excells as it's one of those swords which is almost as good as it's far more expensive counter-parts. This doesn't need to change, that's my opinion.