If it’s likely to get water spray and these are the choices, I would choose the 6. The display is replaceable while keeping the logic module. I've replaced several screens in a washdown environment. A logic module hasn’t failed yet. A display swap is quick and easy.
I would choose the 7 for a dry or damp area. The plastic cam locks seal the unit to the panel well enough, but I don’t perceive it as secure as the screw clamps of the 6. It’s the current model and cheaper. A backup of the application on a USB stick can be copied to a new unit, provided the firmware version matches.
If you’re new to FactoryTalk View for this, I expect you to quickly develop a strong dislike for it. I’m getting used to it now but it really makes me appreciate others.
Consider proposing a PV800 as an additional cheaper choice to the customer, especially if using a ML1400. I like it as a simpler cheaper alternative to 6/7 where it can be used. It’s developed with free CCW. Unfortunately the largest size is 10.4”. It doesn’t have all the features like parameter passing but for simple I/O interface I think it’s great.
Know how you want to connect, serial or ethernet when ordering. Either works fine.
Second guessing myself...the 6 doesn’t withstand direct spray very well. Maybe the 7 would be better?