Depends which is more important to you, the style and laid back elegance of Celebrity or the itinerary of Royal Caribbean. I would choose style and elegance every time. You may find yourselves disappointed with RCI whatever their itinerary. Of course, price may influence the final decision.
Of course the ship must be taken into consideration but it is always over-ridden by the itinerary. You can be taken in a Rolls Royce to the Manchester Ship Canal but it won't come up to a trip in a Mini to the middle reaches of the Thames. I will add that there are other ships and itineraries which are worth considering too. ..... Neil.
In a nutshell it probably depends if you're looking for formality, elegance and a more refined style (Celebrity) or a ship with lots of activities to do on board, catering more for families (RC).
In january there is very few kids on the freedom as they are at school, i sailed the eastern and western caribbean last april on the freedom and loved it. But if you do want more classier nights and a more laid back cruise book celebrity but if you want loads to do and great itinerarys go for RCI.
RCI are 'mass market' and Celebrity are 'upscale'. The former should be cheaper.
Why not look to see what other cruises are departing from same port, I gather Solstice is leaving from FL. Do one week on solstice and then a ship switch for the other week., Fort Lauderdale is a popular port I am sure there are many other ships /lines to choose from. Even if it means an overnight in hotel in between.. It is possibile as this is something we have done in the past on two RCI ships leaving from same port. Just had a quick look plenty to chose from eg: Carnival Freedom, Oasis of Seas, even Celebrity Eqinox.
I have a bit more time on my hands, so I've just had another look at your options: It sounds as if you are really set on trying Solstice. First Week you could do HAL Eurodam, or RCI Oasis of Seas both of which dock back in on 9th, 16th Jan Spend 1 night hotel then embark Solstice 10th or 17th for further week. Or Carnival Freedom which docks back in 10th, 16th, 24th with switch over to Solstice. All of which would give you a good mixed itinery. OR Celebrity Equinox (newer version of Solstice is doing 10 or 11 night itineries out of Fort Lauderdale.