This is a question I often ask myself! Over the new year break, we took a trip to Colwyn Bay (North Wales), and found the derelict pier which is soon to be torn down. Usually with a run-down building, or anything with great textures and shadows, I would go for a mono conversion (done in Lightroom 5). However, the sky was such a great colour that day, it seems a shame to lose that aspect of the shot.
Both taken with a Nikon D90, Tokina 11-16 mm f/2.8 @ f/22, 1/15 sec, tripod.
I did end up posting the mono version to Flickr, but I'm interested to see what the general opinion is!