No idea. I don't foresee us leaving making as big a difference as some would claim, but at the same time it probably will make a difference. I'm more interested about what happens to the UK if we leave. But then, in reality no one really knows.
I'm going to vote in because I believe more integration worldwide is better than isolationism, free movement and the ability to work anywhere within the EU is a major benefit which we may or may not get if we left and I don't believe we will be any/much better off economically if we leave, but we could end up significantly worse off if things don't pan out the way Brexiters want. I also don't trust the current government (or particularly the previous labour government for that matter - the condems were a better alternative to both!) with regards to environmental, health, transport and human rights so having another organisation with some input on those things is a big positive.
Do I want a "United States of Europe" - no, at least not yet. Perhaps I'll change my mind in future, I don't know. Do I think it's heading that way? Perhaps, but not in the foreseeable future.
I'd be more likely to vote out if there was the option of a second referendum that could put concessions and deals after the two year timeframe to another public vote though. By that point a lot of questions would be cleared up (especially on the main points according to the public polls - immigration and free movement) and we would have a much more informed opinion on the matter.