If you flatten the file, all your editing is lost. In that the edits are merged into a single layer. Therefore if you have the room, keep the unflattened file. But if you are sure there is no reason to ever go back to that file to alter it, then flatten away.
Hello, storage is cheap, you never know how you would want to change the file with future tools and enhancements... Keep 'me layers. You can merge some, win a few MBs here and there if you replace a copy of a layer with blending modes by an adjustment layer... But flattening is a one way street!
I find it helpful to always keep a "master' file that contains all the layers, and then flatten a version as needed for clients or other purposes. You just never know when you need/want to go back and re edit.
I think what you asking is if you flatten your psd file before saving as another file type will it be smaller than if you did not. I just did an experiment with saving a very large psd file as a jpeg before and after flattening it at the same quality and the files were exactly the same size. So to save your psd as another file type (jpeg at least!) there seems to be no reason to first flatten the file.
You can not compare two jpg files as jpg always flattens when saving as it does not support layers.
When comparing two psd files or tiff files additional layers will always increase files size as each layer adds data.