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More than likely you'll see way more use from the 24mm TS-E than the 17. The 24mm is the better lens overall and it's more often the right focal length than the 17, which is really really wide. The problem with shooting rooms especially, is that you don't know when you're going to need both of them, and if you're at all picky, you probably won't be happy with a t/s + converter. It works, but at the expense of resolution and more importantly, distortion. Of course, either lens is over two grand, so it may not be within your budget. In the long run, if you can afford it, you won't regret either lens - or both.