I actually think you are both right. Cindy was furious at Casey for stealing from Cindy's Dad's nursing home account. I think that was the last straw! Despite Cindy's testimony that she and Casey lovingly cried over the video of Caylee with the GreatGrandpa and Cindy saying it might be the last time they saw him, it was discussed that a neighbor had seen them fighting in the driveway. Some even speculated that Casey took Caylee and left that night, although they didn't say that in court - but stuck to the Monday, June 16 date as the last time anyone saw Caylee.
I think Cindy was laying down the law - Casey was going to be in trouble and she was going to take Caylee from Casey.
I truly believe Casey is capable of murder just to spite her mother, and to get back at Cindy for not letting Casey go and do whatever she wanted to do, in other words to have her beautiful life.