Difference between flash drive and hard drive?
The primary difference between a flash drive and a hard drive is the speed at which the computer can read stuff off them. Flash drives are faster than hard drives, because of the way they work, so if you buy one of those your computer will boot up quicker when you turn it on and you'll likely find your apps load quicker, too. So, on that basis, the flash drive is good and will make for a better MacBook Pro.

However, if the main reason you're asking is because you don't want to lose files when your drive fails, then what you want to do is buy an external drive. Get one that's twice as big as the drive in the MacBook Pro that you're buying, and use Time Machine with that drive. That will help protect you against data loss. If you don't know where to start looking for external drives, two popular manufacturers are Western Digital or Seagate. The reason I'm recommending this is because neither the flash drive or the hard drive you're asking about are impervious to data loss.

So, to summarise: if you have the money, get a flash drive and an external drive. If you don't, get a hard drive and an external drive. But you should definitely get an external drive.