I went with the Narex 3mm:
I got the chisel today and have been in the shop trying it out. My concerns about flexibility were addressed in the design: It is significantly thicker than it is wide. I did not notice any deflection or bending.
I was able to successfully square off the ends on my practice box joints. I found I could keep my cuts accurate by scoring a line where I wanted to place the chisel, then placing the edge on it to make the cut. I think I'd get a little more accuracy with a jig the back of the chisel would register against, so the chances of accidentally twisting the chisel are minimized. I'll probably try that when I go to build my case.
Flattening/Sharpening was decent out of the box. It took just a few seconds on my Work Sharp to put a mirror finish on the bevel using the highest grit paper. I'm used to needing to start at 120 and slowly work my way up, but this was ready to go.
After about 25-30 operations, I noticed that the closer to 90 degrees the chisel was the cleaner and easier the cut would be. Also, it did not feel like the chisel needed shapening.
This is a tool you can do good work with. It won't hold you back, but it also doesn't push you along either. I wouldn't buy one in this size if I didn't need one, but if you need one it's worth considering this one.
Puckdropper