I’m very curious about the use of such cloud finder. But, to answer your question, the Mask RCNN model would be useful if you want to identify individual clouds. As in, find clouds surrounded by empty sky. Or, for example, you want to find clouds that look like a puppy or or a sheep. On the other hand, if you don’t care for identifying individual clouds and only want to find the pixels that belong to the ‘cloud’ class, then a semantic segmentation model would likely do better.

Also, while I don’t know much about your use case, using DenseNet seems like it might be an overkill since you only have two classes, cloud and sky. A lighter network might give you better results, especially if you don’t have a lot of training data.