The advantage of BBM are that its free and comes with your BB OS. Other than that it only integrated with other BB users. BeeJive will allow you to integrate with multiple simultaneous IM clients (AIM, MSN, Yahoo, Google) which BBM cannot do. That allows you to chat with anyone on a windows mobile phone (I know there are some out there somewhere) or on a PC which BBM will not allow.
It really is a matter of what your needs are. If you need to chat with people who do not have a BB then BBM will not meet all your needs and BeeJive is going to be the way for you to go.