Do you feel desktop last longer and more reliable than lap top because it doesn't get moved around so much ? I've always thought Mac for their reliability and doesn't break down easy. Is this same for lap top ( MacBook/pro/air)?
Are you interested in a PowerPC machine or only the MacBook, MBP, MBA that you mentioned? Apple builds nice laptops, but, as mentioned, there have been some problems with specific models.
A tower is easier to fix if something goes wrong. If there is a GPU failure, swap in a new one. If there is a CPU failure, swap in a new one. Display failure? Plug in another one. You get the idea.
The intended use should generally determine the machine needed. If you need portability get a laptop or tablett. If you do not then look at a desktop. If you do not need portability then a desktop generally offers more value as it is eaier to fix, upgrade, etc.
That said, you can get G4 iBook laptops for around $75 and G3 iBooks for around $35. That might be a cheap way for you to see if you need a laptop or not.