I don't see the Suminagashi in 270, but there's a good chance you can special order one from JCK.
The two knives are very different from your current Mizuno, for example
240mm Hontanren is 3.1mm at spine
240mm DX Honyaki is 3.9mm at spine
240mm Suminagashi is 2.6mm at spine
270mm DX Honyaki is 4.3mm at spine
270mm Hontanren is 3.1 at spine
It's hard to justify the AO/DX price. The special forging process and, essentially, a damascus finish raise the price substantially. B#1 steel is a special steel but not as hard to quench or temper as Super Blue and should be reasonably affordable. Honyaki B#2 should have equal edge retention and arguable a better blade.
I look forward to hearing others opinions on this, as I'm very interested myself. Please correct me if I'm inaccurate. I have not handled these knives myself. I did a bit of research and spoke to some knife owners recently. I went for the 240mm W#2 Honyaki. No notable difference between the W#2 and B#2 grind or profile, and very minimal difference in edge retention. Honyaki knives generally have good retention, and I will mainly use mine at home in a rotation of gyutos so the W#2 made most sense. The white steel will be a joy to sharpen!