2 Job Offers! Which One to Choose?!

I'm a new grad RN and I interviewed for a med-surg position at Hospital 1. I got the job! I received an offer letter, signed it, and turned it in. The same day I turned it in, I got a call to from a bigger, nicer hospital (we'll call it Hospital 2) that wanted to interview me for an ER position (by the way, obtaining an ER position has always been my goal). So here's my question: In the event that I ace the ER interview and get an offer at Hospital 2, can I go back on my signed offer letter at hospital 1? Is the signed offer letter legally binding?

Any advice/insight would be greatly appreciated.

Thanks
If it's a standard offer letter, yes you can "go back on it" but I wouldn't expect to ever be hired there again. Burned bridge. However, some new grad offers are like contracts where you agree to work for two years or whatever, and some of those can have penalties (fees) involved with breaking the contract. The offer letter would state this if it were the case, though.
I am in the same situation right now. I got a job offer this morning and accepted it over the phone then got a call from another bigger hospital for an interview. I acceped the interview, but am not sure what to do. What ended up happening with your situation?