Firstly, where do you see yourself buying eventually? North or South of the border?
You might get a better deposit interest rate in the North, and since Sterling has been falling against the Euro you'll get more pounds now than you would have done previously. Of course it's possible that Sterling might continue to get cheaper meaning you'd see your savings eroded if you view them from a Euro point of view. Are you prepared to engage in currency speculation?
You could always hedge and split the money 50:50 either side of the border.
But if you know where you eventually want to buy a home that might be the best place for your deposit.
I'm not sure of the tax implications of cross border saving and this might be the deciding factor.