When to share fields between content types?
Generally if you happend to have same Field settings (Cardinality etc) you can re-use the same field. But if you want different Field settings (Not widget settings) you have to use different field.

If you have huge number of users(couple of millions) or contents it'll not be a best solution to re-use same field, because field_date_* and field_revision_* tables will end up having a lot of data and that might give you a bottleneck on performance.