I would recommend btrfs at this point. I use it for my OS harddrive and I would also use it for my second harddrive except I don't want to format it, so it's still running ext4.
Theoretically btrfs runs slower then ext4 but you won't see it outside of benchmarks. Btrfs can also do integrity checks and healing so your drive (might) last longer, ultimately, that one depends on what exactly is wrong with it.