I'm in the process of purchasing a 250mm lens for my Hasselblad system. I will soon have a 50mm, 80mm, 120mm and the 250mm. The 120 is my favorite lens for the kit, but it just doesn't have enough "reach" for some of the landscape scenes I want to make. I often want to isolate parts of a landscape or better incorporate distant details into the image.
Since the 250 is roughly twice the focal length of the 120, I figure it will best fit my needs. A 180 would not be that different. (I have borrowed a friend's 180 for a couple of portrait shoots. It is a really great lens!) When you get into medium and large formats, you start to realize that increasing focal lengths require a whole lot more lens than 35mm to create similar telephoto effects. A 250mm is roughly 3X normal for 6x6 cameras. In 35mm terms, that's only about a 150mm lens. My point is, that even thought the numbers seem big, the actual increase in effect is not that great. The 250mm is relatively compact, and isn't that heavy. It's mostly empty space surrounded by metal. The 350mm starts to get ridiculous in terms of size and weight.
If you can borrow or rent either or both lenses, I'd do that before spending the cash. I've happened to fall into a reasonably good deal, so I'm pulling the trigger.