Sigma 170-500 or 50-500\n\nI am pondering whether to purchase one of these lens for my Canon 10D\n(hopefully a 20D soon) and was wondering what people thought of them if\nthey own either one.\n\nI already have a Canon 28-135 so I am sort of heading toward the 170-500\nalthough it is a td slower than the 50-500...but 0 cheaper.\n\nGood lens ????\n\nWhich One ????