Both can happen but a sudden change in decision making especially if it is impulsive can also occur in bipolar and since the spectrum from bipolar to bipolar with psychotic features to schizoaffective (schizophrenia with a mood disorder) has some similarities some symptoms can occur in both. Its a complex question as to what is the exact diagnosis and this should be discussed with a psychiatrist and any new symptoms of concern called to their attention.
that's a good question i think that being psychologically unstable is the word for it like iladvocate said the spectrum of bipolar to schioaffective :)
Changing one's mind frequently is normally impulsive, and more mood-related. It would be more likely to be a diagnosis of schizoaffective disorder or bipolar. Delusions can go with schizoaffective disorder or schizophrenia.