I must first say that I haven´t done any RCCL cruise so my answer is based on that I´m a NCL-lover, so that you know that!!!
I like NCLs Freestyle Cruising concept with no fixed seatings and lots of restaurants to choose from, the Jewel has 12 restaurants and most of them are included in the prize. I haven´t cruised on the Jewel but I have a Jewel cruise booked for next year and I´m really looking forward to that.
Another thing I like about NCL is that they seems to be for people who don´t need to do a lots of things when they are cruising, for me cruising is more about doing as little as possible! The Jewel has no skating rink, climbing wall or bowling alley but there are lots of places to relax and read a book or have a drink. I think that RCCL is more for people who needs action all the time, if you wants that maybe RCCL is a better choice!
NCLs Western Caribbean itinerarys are also very good. I´m not sure which ports they have on the 5 night cruise in January but I have been to Grand Cayman, Cozumel, Belize and Roatan in Western Caribbean and liked all ports.
I hope that this was at least a little help, good luck with your choice and have a great cruise on NCL or RCCL!