Need advice regarding buying unlocked phone in US for use in South Africa

[tiffylocks]

Hi all.. I am new to this forum, so apologize if this topic has been covered. I will be travelling to the US and would like to purchase an unlocked cell phone to bring home with me to SA (either HTC one or Samsung Galaxy S5) I will have to renew a data contract (probably with Vodacom) but not sure if I will have any trouble using a phone from another country on the network here? If company in US says phone is "unlocked - Only compatible with GSM carriers" will I be able to use it in South Africa? Thanks for your advice! PS - if anyone has comments regarding HTC one vs Galaxy S5, I welcome that too!!