If it comes to a choice, the one I would go for would be Disk Warrior.
Disk Warrior is a directory rebuilding utility and has save many a HD from an early demise. It only does the one thing but it does it wonderfully.
Tech Tool Pro is more of an all round player that will allow you to perform a number of tests on your machine. It does a good job at what it does.
So, Disk Warrior first and then on to the Free Stuff. Check out Onyx and Yasu, google for them. Both are utilities that perform the Unix Cron's that are normally done by OS X when you are asleep and the machine is turned on. (Like overnight). There are Daily, Weekly and Monthly cron scripts that you can run whenever you feel like it. The cron scripts check a whole bunch of things on your machine.
Hope this helps
I have a PowerBook G4 with an (almost full) 80 GB drive and I just purchased a 500GB LaCie Big Disk Extreme Firewire External Hard Drive. I don't have any 3rd party disk utilities, but I'm wondering if I should get one. I know that DiskWarrior and TechTool Pro both are highly recommended on this site and elsewhere, but I'm not sure if I should buy one or both. They each cost about $80 and I can't tell the difference between what they do. Can someone who has experience with each please help clarify the differences between the two and let me know if they recommend one or both? I am not currently experiencing and hard disk problems (knock on wood), but I know that it's probably just a matter of time.