Not saying that Anquan Boldin is not a very good WR. But lets not forget that while he does put up big stats, his numbers have come in an offense that throws the ball 600 times a year.
Everyone discredits Roy Williams stats in 2006 because he put them up in Mike Martz's system. Well Arizona thew the ball more times each of the past two years than Mike Martz did in 2006.
All in all, it is six of one, half dozen of the other. With Anquan Boldin, you get a guy who is maximizing his talent level, and is a great possession WR, but you do NOT get much big play ability at all. His career yards per catch is 12.9, with 3 seasons of 12 yards per catch or less. With Roy Williams you get a guy who has yet to play up to his true potential, but even not living up to his true potential is very good. Also with Roy Williams you get a guy who's career yards per catch is 14.5, and has only had one season with a yards per catch under 13. And three seasons with a yards per catch of over 15.
Both are also injury prone, with Roy Williams being just over 1 year younger than Boldin.