One of the allures of Tumblr I think is that you appear to get a lot more response to posts. Tumblr enables a quick "like" and reblog function which can get your posts spreading around a lot via people you don't know. As I've been playing more in Tumblr and thinking about the LJ vs. Tumblr comparison, I've begun to think that for those of us who are not fandom's writers, but it's visual artists, Tumblr may offer a bit more feedback satisfaction. And then there's the fact that for whatever reasons, Tumblr is so much more active for SPN than LJ seems to be these days. Even if the activity might be a bit OTT or youngish or whatever or Cas vs. Sam focused - at least people are reacting and talking - as opposed to the silence that seems to have descended over LJ.
Don't get me wrong, I still see LJ as my fandom homebase. My real community. I'm just venturing out further and exploring and bringing back. :)
Tumblr is not the place for in depth conversations or getting to know-you's. I am seeing that relationships form there and perhaps it's just a matter of getting used to the different behavior of Tumblr, but it still does seem to me to as good as LJ for forming a more "connected" community.
But there is something to be said for finding two of my photos on Tumblr with more likes/reblogs than I've ever seen on LJ.