iPhones and iPads are wonderful devices for both low vision and totally blind users. If you need to make phone calls, then you will need an iPhone; iPads can make phone calls, but only if the user's iPhone is nearby. The most significant choice factor for most people with low vision is the screen size and they may prefer the larger iPad screen or the larger iPhone models, while people with more severe sight loss who can't easily read the screen may prefer the smaller and more portable iPhone. People with severe sight loss will only be able to use iPhones and iPads if they make a significant effort to learn VoiceOver. See my page on selecting an iPhone for people with sight loss for more information.