With the latest Z1 firmware, this workflow by @Juantonto may be of interest to you.
If it is a still, non-moving shot or very little movement, then merging with Lightroom is giving some great shots. As far as I know, the THETA stitcher is fine and is at least as good as PTGUI.
I believe one advantage of PTGUI is batch processing. You can also use the THETA stitcher standalone if you don’t want to use lightroom