I think 16' will serve you better if you have a family of 5. If this is going to be your only boat, you could go 15'5". What you should consider is not just the length, but tube size. NRS Otter 150 is only 20" so it won't have as much carrying capacity as a 22" tube. I have family of 6, but two of them are still very young. So I settled on a 15'7" with 22" tube diminishing to 19" in the bow and stern to give more room for passengers and cargo. Even then, I see the boat serving our family for just a few years. Once the kids get older, they will get a ducky or a smaller 13' that will double as my fishing rig.
Even 16' will be crowded with 5, but if you pack judiciously something smaller could work. Agree with the previous post. There is little difference between 15' and 16' and if anything, larger tube will give make it easier to maneuver when loaded down.
It's awesome to take the kids out early on. Much better than sitting in front of TV or video games. Get something that will get the whole family on the water, and enjoy the ride.