None of those arrangements are "backups". They are disk arrays with redundancy to keep the array running if a hard drive fails. Despite sounding wonderful, it's entirely possible to lose data or have corrupted data on a redundant RAID array. I've seen it many times.
Getting a warning of a drive failure is one of the issues. Cheap built-in and add-in RAID cards don't have audible warnings. And they don't have built-in SMTP mail servers to send warning emails, either. Expensive RAID controllers will have audible and emailed alarms available if configured correctly.
There's also the distinct possbility of having more than one drive fail at a time, which I've seen three different times in three years.
If all you want is backups, then set up an automated backup system for important data. There's lots of good software and hardware for doing that nowadays. Preferably, keep more than one set of backups and keep a set offsite (either physically, or via web-based backup services).