Sweety, I have been a member of BASSA within three different airlines and found them really good. CC89 I've never experienced, although heard a few bad reports about them a few years back, but that could've been chinese whispers, or any problems could be sorted now.
Most of all, does your company have union representation within either union? If not, find out what you will be gaining as an individual employee member, it may not be worth it. In my experience the best thing about being in is if the airline recognises the union and you have union negotiated agreements on matter such as hotac, seniority, etc.
Hope this helps (I doubt it!) and good luck with whatever you decide
MR XxX