Well, they are very different and for numerous reasons, but too many for me to go into detail. AP flour is made from soft-wheat and semolina or durum from hard wheat. In Italy, the classic fresh pasta made in Bologna is made with soft wheat flour and is generally the most commonly used at home. Semolina/durum has higher gluten and is harder to work with in home applications. It is the flour used most commonly for commercially extruded shapes. I prefer to use all-purpose, unbleached. It will be interesting for you to work with both and see which one you prefer and post back results.
Now... I can point out that almost every italian chef will tell you that in making BASIC fresh pasta with AP flour, nothing else goes in the dough other than flour and eggs (chopped spinach for green/squid ink for black in colored pasta). In semolina dough, often just water and semolina. Salt should be used in the cooking/boiling water, oil makes it slicker and is undesirable and nontraditional. I can point you to Ada Boni, Marcella Hazan, Giuliano Bugialli and Mario Batali as good sources of reference.
I have used Mario Batali's recipe to great success at a restaurant:
3 1/2 cups all-purpose, unbleached flour, plus extra for kneading
5 large eggs
Use the well method. Knead, knead, knead, knead some more... 10-15 minutes (you can use your own method if you are not kneading by hand.) Wrap dough in plastic wrap and rest for at least 10 mins before proceeding.
(NOTE: the absorption capacity of the eggs and humidity in the kitchen or lack thereof may cause slight variation in the amount of flour needed)
Buon appetito e buona fortuna!