The repairs are similiar, but more limited when you are working on the boot disk, and more dangerous too. Basic things like permission repairs and such are ok, but if you need to repair directory structures or B-Tree headers for example, it is MUCH safer to do them while booted from another HD or an OS X CD. DiskWarrior, the #1 app for these types of repairs, will NOT allow them to be done on the boot disk at all.......
FYI, there have been recent mentions that Apple has been (slowly) incorporating/adding some DW-like functions into DU, and that they may come full circle in the Leopard version.......
Some basic file system repairs can also be done while booted into safe mode too (hold down cmd + S keys during the boot process)
The difference is that Disk Utility won't let you repair the boot volume, so you have to do it from the OS X install disc (or another bootable drive with DU on it).