im considering getting a 70-200 f4L as an upgrade from the kit lens. should i get the f4L non-IS for abt $860 or the 70-200 f4L IS for abt $1600. is the IS really worth twice the price? i wont need the weather sealing as my body isnt weather sealed either. I mainly shoot portraits and events so is the cost of twice the price worth to get the IS? any advice?