skymaster

Hi, I am an international applicant for chem engg, phd program starting fall 2010. I got accepted and sent an email accepting the offer from a 43rd ranked univ. Now I have just got an accept with similar financial support from a very good univ (ranked 18th). Can I and should I accept the latest offer? I already have the hard copy offer from the 1st univ but haven't signed or sent them as yet. I doubt if there are any legal issues (are there??) but it seems wrong anyways. Thanks, Akash