It depends with the scope of your service and your customer's needs.
For Example:
1.) If you want to use bulk sms service for your customers to market their products & services to their target market then you must consult with your mobile operator of choice who can then broadcast the sms's to their subscribers.
2.) If you want to use bulk sms service for your clients to communicate with their clientele then you can use either econet or the web based bulk sms service. I used
http://www.bulksms.com/ for a free sms site and generated $ from advertisements. With BulkSMS they provide solutions to send and receive SMS messages using the Internet through:
BulkSMS's Desktop Software
The BulkSMS Web site
Your Email Software
Your Mobile
One of BulkSMS's API's
The BulkSMS application programming interfaces (eAPI's) are easy to integrate, making it the best choice for developers or any company to SMS enable own systems. E.G. You could subscribe the service and link it to the Bank system so that any transactions done on one's bank account will send an sms alert. You can use the same for various uses etc.
Compare the prices from the available mobile operators i.e. ECONET / TELECEL / NETONE and the international community and make an informed decision.