The Ukraine will essentially lose sovereignty if it joins the EU. Its economy will be “managed” by the European CB. I am well acquainted with WW2 history, and I am only offering a parallel.
The real problem is that the world is on the march to global government and the major players are consolidating their gains. The Ukraine is just another chess piece to be taken.
Ukraine is a sovereign country that can opt to join whatever organizations it wishes. Here's the stark contrast between the EU and the USSR: EU has no army to occupy its neighbors' territory. RF has, and just did.