Both are open world as we know. You see they can be compared but they can't be in a way.
I thought it would be somewhat interesting to compare the very best detailed two open world games at the time even they have completely different playstyle.
I compare them as one being far more detailed to interactions, graphics and storyline but a far smaller restricted world with incredible detailed introverted gameplay and lifelike interactions but I feel like it's less dynamic in a way. (Shenmue)
III has more freedom in a larger world, more dynamic and chaotic world but less graphically detailed. It certainly feels more like open world and detailed in an extroverted way.
A minority said it shamed GTA III in terms of graphics but I personally don't because GTA III is more extroverted in great detail and had to be that way for a larger scale world.