When it comes to economics, there are always winners and losers. This is the case even at the most mundane of levels. So when someone says that “X will lose out”, that means that Y will win-out. This is true about all economic policies, from Stalin's to “Austerity”.
So, in the end, there has to be more to this debate that economics. EU is filling the UK with masses of recalcitrant immigrants. It's taking away our rights with one hand and creating laws that suffocate us with the other hand. In the end, I see the “democratic deficit” to be far more important than economics because the economical reality always rides on the back of political changes – contrary to what Marx argued.