Question on Kindle Return Policy?

Ok, i got my first kindle on christmas last year in 2010. In February of this year it broke. I returned it a week ago after I was sure I could not fix it myself. They sent me a new one a couple of days ago. Last night i was reading, and it suddenly switched off on me. It was fully charged. When i tryed to turn it back on half the screen stayed the way it was before i turned it on again. it still had a picture of half the background in sleep mode. The rest was perfectly fine. I was wondering if i could return this one a second time and get a full refund. And a second Question: Is the nook better than the kindle (not the nook color but the other one) Thanks in advance:DD