three points to consider:
* don't consider porting unless your contract is near its (24 month?) expiry date
* read the contract fineprint: despite the contract expiry date having passed, you must give notice, sometimes three months in advance. if not, you may get billed for the remainder of the notice period.
* porting has its pains; the process is by no means sorted out properly. There may be long delays, and the cellphone outlets generally are not well informed about porting. But once done, it is OK (When a VM subscriber phones me, they get a stupod "empty, empty" voice recording before my phone rings - one of the porting niggles)