Halp! I am going to Hawaii next month, and I want a ultra wide to go in my bag with me...I am torn between the Tokina 12-24 f/4 or the Siggy 10-20 f/4-5.6 I do shoot weddings alot, and this would be a great addition to my bag of tricks, but does the extra 2mm on the Siggy, worth the speed? The constant aperture of the Tokina appeals to me, and with better build quality too. Anyone have any suggestions? This is for Nikon mount.