I own the C9000 along with 6 Sanyo 2700 and 4 Sanyo Eneloops. It has been working great. I have bought the charger and the batteries to power my Zoom H2 (bad recorder, stay AWAY!!!) and all the batteries have been working great. It has all the regular modes (charge, discharge etc.) and also has cycle mode that can be used to revitalize old cells with high internal resistance.
You should take these few issues into consideration before buying:
1. The BC-900 is smaller than the C9000.
2. The BC-900 has a very small screen that is divided into 4 parts, one for each battery. The C9000 has a large backlit screen that shows all the stats about one battery at a time and then moves on to the next cell.
3. In the C9000 there is a bigger space between the 4 cells for better heat dispersal. In the BC-900 they are pretty packed together so the batteries may heat up during charging/discharging. That may eventually hurt their performance.
4. The BC-900 is cheaper by about $15.
If you go into the link that I provided for the C9000, there is a photo that someone added of the C9000 and the BC-900 side by side. It may help you understand what each unit is like.
Before I bought the C9000, I researched a lot on another forum (I am not sure if I am allowed to post its name here. In case it is allowed I will add the address) and over there everyone said that the C9000 is the best way to go. I am not saying that the BC-900 is not a great charger as well but I decided to follow the people and I am not disappointed.
I think you will be happy with what you get, whatever choice you make...