Since ureadahead looks at blocks and not files, it has a potential speed advantage if the boot process is opening some large files but not reading them in full.

Edit: except the ureadahead packfile only points to the blocks and files, it does not provide a way to inline them. So e4rat is almost certainly faster. It's a shame it is ext4 specific.

Apparently Scott James Remnant, the ureadahead developer, considered feeding the collected info to a defragmenter: http://ubuntuforums.org/showthread.php?t=1434502 ; this would be nice, as it means a single package is responsible for the feature, and filesystems perform to the best of their ability whether or not they have ext4-like fine-grained control of defragmentation.