A lot of interesting opinions. I'd have to say, when it comes to writing longer pieces, I always go with writing that interests/inspires me the most. After it's done, I sit down and ask myself: well, who is going to read this? And another: who is going to enjoy it?
On the flip side: I've set aside writing exercises in the form of short stories--these with a definite agenda in mind--a different genre, a different setting that requires research, a different narrative point of view, writing about characters that normally I wouldn't put in a story, to try to challenge myself and broaden my scope as a writer. These are harder to put together, but definitely worthwhile.