Everyone has different tastes. I'm still new at coils, but have successfully built a few for my kanger aerotank which I believe is the same atty as the protank. Unfortunately until I get some new Kanthal all my builds seem to only make it sub ohm, about .8 to .9. Not a lot of room in that chimney.
I actually would like it to be about 1.4. The downside to sub ohms is that just about all regulated devices can't handle it. I do run my sub ohm on my mod and it works fine. But my main reason for trying was 1. Cotton wick; and 2. To try and get a little lower resistance. But I want it to run on my MVP and below 1.2 the MVP gets weird. Too low and it doesn't work at all.
Anyway,to answer your question, I'd say it's all about preference. But again, I do wonder if it will catch on due to the fact that most regulated devices won't handle it.
I run 1.0 coils on my MVP with a Kayfun and never had an issue , haven't tried lower than that though.