What is better for bones? Calcium carbonate or calcium citrate?

Either ok. Either form of calcium is reasonable to take as a supplement. Choose a form that you won't mind taking as some are definitely more palatable than others. Calcium citrate may be somewhat better absorbed as calcium is best absorbed with some acid. In general, what is more important is to space out your calcium consumption throughout the day as possible since if you take all your calcium at one time it is not well absorbed.