I'm for Remain.
I'm mainly concerned with the economic impact. There's pretty much universal consensus amongst serious and credible sources that the impact of leaving will be somewhere between very bad and catastrophic. World Bank, Treasury, IFS, IMF... apparently when Beexit guys try to find an economist who will appear to offer the counterpoint on the media, they barely have a choice of 1-2 minor figures compared to the hundreds o the other side. Anyway... loss of direct access to the single market will really hurt exports and trade right away, which will cause a recession. Fewer jobs, reduced or negative growth. In the medium term there will be a significant decline in inward investment. So yet again, fewer jobs, less growth. The "we can get a new/better deal quickly" is just a lie.
I'm not much moved by the sovereignty argument. Sovereignty is not binary, something you have or don't have. It is a matter of degree. In fact, almost every country has given up its sovereignty to some degree. Members of the WTO give up sovereignty over some aspects of trade policy. Signatories to the land mine ban give up sovereignty with respect to national defense. Members of the UN have more or less given up sovereignty insofar as they are bound by the UN Security Council. The list of ways that countries have given up sovereignty goes on and on. The point is that almost all countries find it useful to bind themselves in some way if others are also bound. So it's not "sovereignty above all other things" it's "sovereignty on all issues where it's not worth giving up sovereignty".
So the sovereignty argument more or less collapses back into the argument over the net benefits of being in the EU. If the net benefits of its are sufficient (and the benefits are mainly from trade plus the upside of freedom of movement), then it was probably worth giving up some degree of sovereignty. (Not all of it, because it is a matter of degree.)
If the issue is that you see the EU as not very democratic, well, I'm not an expert on this at all. But my understanding is that over the last 15 or so years they have improved things. In particular, the Lisbon Treaty made lots of helpful changes like increasing the power of the European Parliament (for which there are direct elections) relative to the Commission, and increasing the role of national parliaments in enacting European legislation. And to the extent there's a deficit in democracy, I would rather we were in it, pushing for further reform, then outside.
