I'm not sure we'll get much meaningful discussion here. It's like having a debate of Trump vs Hilary vs Bernie.
I guarantee if we had a land border with Europe, someone on the leave campaign would be talking about building a wall :D
If Trump is elected, he may have to build a second wall along the Canadian (Canadienne) border - to keep people in the U.S.
And if we leave it open? All the migrants will just flood through NI. The whole idea that we can control migration by leaving is absurd nonsense.