That to me should answer your question. One is 39.4mm and the the other is 41mm so they will fit your wrist differently. Other than that my vote goes to the 5170P for the following reasons. The dial on the 5170P catches the light very well so it changes colour beautifully. The same movement is in both watches but it fills up the case back of the 5170P much better. The 5170P is in platinum which has the edge over the 5172G's white gold case. The 5170P is a future classic in my opinion and the diamond markers are hardly noticeable.