I just goes to show that playing on a good team makes it easier. I'm not saying the Williams is far and away a better player, however I feel that Williams would just be a successful as Parker in the Spurs system.
That said I would pick Parker every day of the week. He development and growth over the past 11 years have been enjoyable to see and he has proved the perfect PG for our team (willing to defer to Duncan/Ginobili earlier in his career and now in the conversation - all be it briefly - for MVP).