Their princles are different. For the monopole, the idea is from the dipole and the length of the developed printed monopole is about wavelength/4, the profile is very low and there should be no ground plane under the monopole. For the PIFA, the length is also about wavelength/4. It should have a ground plane under the radiating patch and the profile is a litter larger than that of the monopole.
hi for mobile antenna design , i suggest you to search one book written by a taiwan person , i am sorry i have forgotten his name and the book name , the book is something like MICROSTRIP BROAD BAND ANTENA DESIGN .
ok good luck