If its any help I have been using my power meter on Zwift with the Bkool trainer just in charge of changing the resistance. After doing this for about 3 months I did a ride using the Bkool as the power meter. I'd say they average out similar but hills were definitely harder on the bkool yet flats were faster. Pretty much the same as I found in the Bkool software. Each Bkool trainer seems to behave that little bit differently as I always thought I was handicapped on the hills with mine!
Just done my first Zwift ride. I found the hills easier than bkool - as in less resistance. However, I don't know if that meant I went quicker - I could just stay in a higher gear. Definitely noticed how much smoother the slope transitions were, but I couldn't see the kind of graphic you get on bkool telling you what slope was coming up - don't know if I am missing something?
I will be interested in trying some races with lots of competitors, although I actually think I would prefer to do a non-race ride in bkool. I think Bkool looks nicer (although that could be just that I'm more used to it!) and the variety of courses and the video rides done at my own pace appeal more than riding in groups for me.
Anyway, I'll be back!