5400rpm drives have higher latency and will be slower.
My suggestion, since you have two spots for drives would be to use a SSD on one, for the os, and parhaps a few games.
Use the other for storage. With a ssd, I would be OK with a 5400rpm drive, but 7200rpm would be faster.
For data security, you need EXTERNAL backup.
The value of raid-1 and it's variants like raid-5 is that you can recover from a drive failure quickly. It is for servers that can not tolerate any interruption.
Modern hard drives have a advertised mean time to failure on the order of 500,000+ hours. That is something like 50 years. SSD's are similar.
With raid-1 you are protecting yourself from specifically a hard drive failure. Not from other failures such as viruses, operator error,
malware, fire, theft, etc.
For that, you need external backup. If you have external backup, and can tolerate some recovery time, you do not need raid-1