The two on the left are called NanoDia, from uni(ball), while the two on the right are the Pentel Ain Stein (read: "Einstein").
The Ain Stein container has a small opening, so I had to give it a few gentle shakes to get the lead out (at least with a full container). The NanoDia has more access, but this also comes with a greater risk of spilling the leads.
The paper used for my comparison was an Ampad Gold Fibre Quadrille pad and the pencil was a Zebra M301 Ultra.
The uni was smoother in both cases, but neither lead had any issue with becoming crumbly or snapping with a decent amount of pressure applied.
I apologize for the bluish color in the photos; it's often quite overcast here in Minnesota during the winter, so the color is not quite accurate, but it's mostly the consistency and shading that people look for in a pencil lead anyway.
While the Ain Stein B and uni NanoDia 3B were both very smooth, the uni NanDia HB was certainly smoother than the Pentel Ain Stein HB. Interestingly enough, JetPens seems to have discontinued the Ain Stein leads, so I'm not sure if it was an availability issue or if the lack of popularity lead them to remove it from their offerings.
As a side note, the NanoDia is supposed to mean small (Nano) diamonds (Dia), so lead breakage is supposedly minimized with the addition of a little diamond dust. Because none of the above leads have ever broken on me, I can't say that the diamonds help, but I can't say that they don't either. I guess I will just have the piece of mind knowing that the diamonds are doing their thing to keep me writing without the period, annoying snap of a broken lead.
uni NanoDia - JetPens - $3.30 The HB is currently sold out, but you can most of the other hardness levels.
Pentel Ain Stein -> not available from JetPens, but you can still get it elsewhere if you're interested.