Worth the upgrade? Dont get me wrong, i really like the Tamron i own. Sharp and very crisp shots with it. But, in some light it does seem to have an odd yellowey look to some of the shots and focus seems to miss now and then. But over all i have been very happy with it. But from the beast owners, i have never heard a peep bad about it, so...... worth the upgrade? Any one every had both in thier bag?