PSD is limited to 2 GB, and Tiff to 4GB. PSB takes on larger sizes, but is not yet widely implemented.
PSD is the native file format for Photoshop and is good if you use other Adobe products. Tiff is preferred if you are sending your file outside and you do not know what your client's software can take. A flat tiff is what one instructor tells me he hands off to the printers.
But if Tiff works in all circumstances and causes you no problems in your work, stay with it.
But why is PSD good if I use other Adobe programs (which I do). The few vector tool for photoshop, like the shapes and text tools are still working in Tiff. : )