I can't compare the two as I've only been to Jersey, but absolutely love it. Beautiful beaches, towns around little coves, or long stretches of beach. Lots of nice cafes and restaurants, and lovely countryside as well. There are a few castles to explore, and some nice gardens, as well as the Underground Museum which highlights the WWII invasion by Germany.
I agree that the hotels on St. Brelade's Bay looked very nice.
I'm a bit biased though, since my Mum was from Jersey, and I have relatives there still.