"Decide on" vs. "decide for"
To "decide on something" means to make a decision in relation to a situation, whereas to "decide for something" means to make a decision in favour of a party in a dispute.
For example, if group A wants to build a monument and group B wants to have a bank holiday, then you could 'decide for group A' (referring directly to the dispute) or you could 'decide on building a monument' (not referring directly to the dispute).
Also:
decide on vs. decide against
(in disputes) decide for vs. decide against