They are both totally brilliant lenses, but I think the 180/2.8 is far, far, FAR more practical. For almost all intents and purposes, their optical performance is identical, but the 2.8 obviously is one stop slower. What you gain by sacrificing this stop is a lens that weighs 970g instead of 2500g, so 1.5x lighter, one that uses a 67mm filter instead of odd series 6 filter in a drawer, and a lens that fits easily in a bag, is easy to shoot handheld and is half the price (or less). Unless you are going to be shooting at f/2 all day long, the 180mm elmarit has so many advantages with essentially no sacrifices. The extra stop does not even really help you with handholding since the lens is so much heavier. And seriously, you must try before you buy. It is hard to convey in words how much larger the 180/2 is. It focuses smoothly and is a nice lens to hold for its size, but the 180/2.8's focus is just as smooth and balances better on most cameras.