[cstreit]

There's no reason it needs to get ugly.

Simply state that you are moving on to a company with more growth opportunities and a better personal work/life balance that best reflects your current objectives. This is not a failure of (current company) but a reflection of the tremendous opportunities offered in the new role.

If they ask you to reneg, you simply state that you have signed another offer and it would relfect upon you poorly as a professional to go back on a business agreement. Thank them for their concern with retaining you, but this is the right decision for all parties involved.