I've been on both the Jewel and the Gem and they are very similar. I'm trying hard to recall the small differences. The only difference that immediately came to mind was that I loved the feel of being on the Jewel a little more strongly than the Gem, but that may have been its itinerary and the fact that we were on a 12 day cruise, while only a 7 day for the Gem. The captain of the Jewel was so obviously proud of the ship and invited some of us for a viewing of the bridge so that made it special too.
Sorry, I can't offer anything more concrete. They're both beautiful ships and I'd jump at the chance to be on either one.