I feel such a fool. I have never played with those settings and didn't realise what they meant - and it seems I am not alone!
Can I suggest you place the default option first and re-word those options as:
(•) Open showing the Main Page
Shows a list of sub-forums (if any) followed by the topics on your forum. Note that if your forum has multi-level sub-forums, you only see those sub-forums that are one level below the current level.
( ) Open in Topics View
Show topics from the forum and all sub-forums in a single flat list, so you can browse topics without having to visit sub-forums. This format is only useful when you have sub-forums.
I think it was the single words "Topics" and "Standard" that confused me as it is "Topics View" and "Main Page" that you see when switching views. (My descriptions are slightly shorter too!)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.