Paying for data that cannot be fully utilised due to poor Vodacom network.

I recently upgrade my contract in April where my initial intention was to cancel this contract but I was convinced to rather continue with the contract and was offered a very good data deal by Vodacom. However I have been really disappointed over the past few weeks as I have had to use a different service provider when it came to data and signal quality which has been far more superior to that of Vodacom. I just feel that I am paying for something that is not really adding value and I have contacted Vodacom twice in connection with this and it seems that they don't care about what they customers have to say especially since there customer in this instance can prove that another service provider has a far better quality of network. I would like to cancel my contract with Vodacom as it is not serving its purpose.