I think I'm echoing some other's sentiments but my wife and I cruised on Celebrity Solstice and greatly enjoyed it. We didn't have our son with us and definitely felt that it catered very well to the 40+ crowd.
Since then, we've cruised RCI twice with our son and we've found that RCI caters much better to families.
I think both give a pretty nice level of service, maybe a touch better at Celebrity. I appreciated the more in-depth learning opportunities that Celebrity focused on (Alaskan cruises) BUT I do think RCI brings a much more lively environment. The various boardwalk events (on Oasis, anyway), parties in the evening, etc. really helped bring a lot of fun, even adult-centered opportunities, to the boat.
If it was just my wife and I, we would be coin toss between the two. With my son, hands-down, no-brainer RCI.