They are the same software more or less.
The per month license
- will stop working if you stop paying
- will get new feature updates automatically
- will be updated for new systems and new apps (like Word)
- includes cloud storage and online services (optional)
- includes some mobile services eg edit on iPad
- is included with the full Creative Cloud subscription
The single payment
- will not stop working unless you do something unwise like upgrading to a new system or app.
- gets major bug fixes but not updates for new systems or apps.
- doesn't get the new features until, after a year or three, you pay again for a new version
- has no online or mobile features
- is something Adobe hope to phase out and don't promote.
- ends up out of step with the subscription version making it hard to follow tutorial etc.