签了一个Offer， 来了一个更好的offer，怎麽办？

在签此Offer前（此Offer不能再延长了），更好的offer正在办。因为此Offer注有：As indicated on the application form you completed, your employment and compensation with *** are “at will”in that they can be terminated with or without cause, and with or without notice, at any time, at the option of either *** or yourself, except as otherwise provided by law. The terms of this offer letter, therefore, do not intended to create either an express and/or implied contract of employment with ***.所以先签了保险，更好的offer现在来了，怎麽办？是不是应该问一问后面的公司介不介意？很想签这个offer。请问各位这样做会不会不好？谢谢！