Better to purchase the device outright, then you are fully covered by manufacturer's warranty and the CPA. Taking a device as part of a contract and the device remains the property of the network until the contract is ended or paid off and exited. They have full rights on the device throughout the contract, i.e. repair/refurbish rather than replace within the first 6 months should anything go wrong. The network "kindly gives" you the device at the end of the contract.
If MTN I suggest prepaid MTN for voice/sms and Axxess for data.