The Z11 has those gills on the side. They have smaller fans (80mm molex) sucking in the air (larger the fan the quieter it is rule of thumb, Z9 uses 120mm) and no dust filters either on these gills, so it's just scooping dust into the case.
The Z9 Plus is great for the price. It also comes with a P4/8 extension to help hide the cable behind the mobo to improve airflow and keep things looking tidy. You don't get the digital temp display or the fan controller on the Z11 either.
I do like the look of the Z11 and while it does have USB3 ports they arent using internal headers so have to be rooted out the back through the PCI expansion bracket. The Z11 does have a dust filter over the PSU intake (Z9 doesnt), the slots for venting the PSU aren't great on either case, I cut mine out.
Lots of pros and cons to consider.....hope im helping more than hindering