You will most likely be satisfied with the a/d converter built into the pic. I've been using them with great success for years on that same PIC. It is likely that your circuit design and board layout can contribute more to the a/d accuracy than the actual capabilities of the a/d. Assuming, of course, that your PIC code is written properly. Either way, you'll have to read the a/d data sheet thoroughly, so it might as well be the PIC's.