Recommendation for filesystem for USB external drive for backups

[rhkramer@gmail.com]

I'm getting closer to setting up a consistent backup plan, backing up to an 
external USB drive.  I'm wondering about a reasonable filesystem to use, I 
think I want to stay in the ext2/3/4 family, and I'm wondering if there is any 
good reason to use anything beyond ext2?

(Some day I'll try ZFS or BTRFS for my "system" filesystems, but don't see any 
point (and don't want to learn) either of them at this point -- I don't see 
much need for a backup filesystem.)

But, I'll listen to opinions ;-)