Buy iDefrag or restore the system via TimeMachine? What's the better/faster way?

Hi there!

I'm thinking about either buying iDefrag or restoring my system.

As I read on several websites there is no need to defrag Mac OS X because HFS+ automatically does (more or less). But: if the free space on the HD goes below 10% the OS can no longer defrag itself even if you delete files and free space increases again. Is that true at all?

As I had only 40GB left out of 500GB I am now thinking about what to do. Is there any need to defrag or restore (iDefrag: 27€ / system restore: just some time)? Would any of those two options speed up my Mac? Or should I do nothing? I'm not sure if the Mac is slower than it should be at all.

Thanks for your opinios and evaluations.