Be aware that if on iOS you want to edit and not just view your outlines you’ll need to unlock/buy the pro iOS app, which I think is $50 now.
I bout the Mac Pro app when it was $50. It has some unique features (like columns) but otherwise is a fairly clunky app with unintuitive ways to create templates to personalize the font/size/look.
I would not pay $100 for it today, and definitely not $150 for Mac/iOS connectivity.
These days I’d suggest looking at the surprisingly powerful free tier of Dynalist’s service, or if you want something Mac/iOS-specific is look at Outlinely or Cloud Outliner (Pro), both of which are in the app stores.