I guess the wired LAN port and regular size USB and HDMI ports could be a plus if stuck troubleshooting, and you dont have to solder in the header on the 3. As far as function goes the zero-w will give you the exact same HM experience as the 3. I've been using the zero-w for a good while now and it works out great, specially for the price.
I don’t disagree that the zero-w is fast enough. All you’re doing is monitoring a bbq sometimes. Maybe change a setpoint here and there. The 3B+ is significantly more responsive though. Like I said, whether that’s important or not is a decision some one will have to make for themselves.
Like I said, my HM is extremely fast with the zero-w, much faster than the older A's and B's. Frankly it's so fast I can't imagine that more speed would be required or beneficial. I think putting doubt about the zero-w speed in the minds of readers of the forum is doing a disservice to HM builders. The zero-w is more than fast enough to run the HM, period.
I have never seen any differences to the various Rpi's. They all perform the same for what we need them to do. If you are experiencing a difference, I would look more at wifi interference, and your network.
The heatermeter case that Bryan designed is customized to fit what ever pi you need. You just tell him which pi your using and if using a thermocouple or not at check out and he'll print the version you need.
If you're printing them yourself you can use the "Open in Customer" option at thingiverse.com to generate the 3d files that you need for your setup.
If you're hardcore you can download the source code bryan wrote to generate the 3d files and tweak things yourself with a really neat 3d modeling program called OpenSCAD.