looking for some more reach over the 70-200mm x1.7 for my D90 eventual D300/400 but i don't want to carry a 500mm prime or spend anywhere near that much $$. i've narrowed it down to either the Sigma 150-500mm or the Tamron 200-500mm. from sample shots i've seen and actual lens copy consistency i'm leaning toward the Tamron. it's also $150 cheaper. but i'm not too familiar with Tamron and couldn't find any review or product descriptions saying if there was any type of stabilization built in. anyone know? has anyone had experience with both of these?