I was in a similar situation not too long ago. There are very few companies that bind you to stay with them for a certain period of time, even if you have signed the contract. What it can also implicate is, they can lay you off whenever they want. So, no reason to scratch your head about this, although I just read today how "scratching helps to block unpleasant thoughts." : D

 But I would advise you to be "nice" to your first employer and give a written notification that you have decided to seek opportunity elsewhere. Be grateful to them for their offer. It obviously is not so good news for your employer, but nothing makes it worse than an arrogant and a rude reply.