OS 10.5 - Backing Up My PowerBook G4 [2004]

Here's what I want to do: Clone my entire PowerBook hard drive [apps and files] onto a Maxtor One Touch Xternal Drive. The op system on the PowerBook is Leopard 10.5.8. 1.5 GB RAM, 1.33 GH Processor. In a year or two, if my PowerBook crashes, because of age, I want to be able to buy a MacMini and clone everything [apps and files] from the Maxtor xternal drive, onto the MacMini, with no probs. Time Machine? Backup app? My friend recommends Backup, but will it be compatible for sure with the MacMini when I get it? Is Time Machine safer? My friend says Time Machine slowed his computer down too much.