Postpaid advantages:
Grace period for payment. Prepaid usually shuts off the day payment expires
Roaming on other carriers, usually at no charge.
International roaming
Discounted equipment offers (e.g. BOGO)
Switching incentives
Financing of equipment
Larger sized family and business plans, may be cheaper for groups
More unlimited options
Postpaid disadvantages:
Higher costs
Taxes and fees
International calling usually more expensive without add-on
Credit checks