Should i buy an unlocked iPhone 4 (450+tax) and put it on tmobile prepaid, OR, buy a Virgin Mobile iPhone 4S fpr $450? I've heard you can unlock virgin mobile's iPhone to use on other carriers, idk

Also I'm afraid virgin/sprints network is ****** in my area and i AM NOT here for EDGE speeds on an iPhone.
Also white or black? Im leaning more towards white but idk ywt..