I go with ext3 for this personally. NTFS doesn't store *nix style filesystem permissions, and causes various other issues with you Linux systems. With ext3, you can store all your files with all of your permissions intact, the filesystem is mature and trustworthy, and you can still access all of the files from any operating system by simply connecting the drives to a dedicated fileserver machine (an older computer or small device works perfectly for this). Simply share your files via NFS, Samba and ftp (if you need ftp access for something like xmbc). Having a dedicated machine for this means you can also script your replication to the secondary drive, so that you only have to attach the drive for the mirroring process to take place.
This is the solution I've been using for about four years now, and it works great for me.