I haven't been on either ship, but am sailing on the Radiance this Summer (Alaska), and what I have seen on the web I am impressed with the Radiance class.
I had the same dilemma for next spring break (March 2016), and I ended up picking the Adventure over the Jewel (Radiance Class), since the itinerary is similar and people on the web convinced me there would be more to do for kids. I'm still second guessing that decision.
I think itinerary plays a big part unless there are just a ton of sea days.