iPad versus iPhone
My opinion on iPad verses iPhone. All statements supported by experience, research, and conversations with other people with varying amounts of vision.
Benefits of iPads
An iPad is likely a great thing for people with some vision. It's great for watching videos or for showing videos to others.
You can have 2 apps on the screen at the same time. This can be useful for using both Zoom meetings and Safari at the same time, or having Notes and another app simultaneously in use. Of course, a 4 finger swipe will quickly switch between running apps on both iPhone and iPad, but in that case they won't both be on the screen and Zoom will pause video when it isn't visible.
Using the built-in iOS magnification feature called Zoom is almost certainly better on the iPad.
You can use larger font sizes on an iPad without having the problem of almost nothing showing on the screen.
Images can be bigger. High resolution images can be zoomed in to see more details at once.
If you use a keyboard heavily iPad OS has some added keyboard functionality over iOS which runs on iPhones. More exactly, more app developers have added hotkeys for iPads. The basic keyboard functionality does exist on iPhones.
Using handwriting or drawing with an Apple Pencil or other stylus can only be done on an iPad.
If you play games that use a lot of screen area and you don't want to have to use extremely precise little movements to explore them an iPad screen is great.
Exploring maps is better on an iPad, but then you can't use them many places without buying the more expensive cellular version, because the WiFi versions don't have any kind of GPS.
The cheapest iPads are cheaper than the cheapest iPhones. This is because less technology needs to be packed in to a larger space.
Benefits of iPhones
An iPhone is almost always way better for a person with very little to no usable vision.
An iPhone is small and can always be with you. This is good because no one should be without their smartphone in an emergency. This is even more important for anyone with a disability.
A huge amount of business, identity confirmation, and communication now takes place only via text message. This means you will have problems if you can't read them. An iPad won't let you send and receive text messages unless you already have an iPhone.
The layout of iPhone screens is simpler than iPad screens. This makes it much easier to learn how to navigate new apps.
The iPhone is like a single car driveway. They usually are straight forward and not easy to get lost in. The iPad is like a large parking lot. They are very easy for a blind person to get lost in. I have spent large amounts of time trying to help people get unlost on their iPads.
An iPhone is useable without knowing the layout of the screen, though it is much more efficient to use if you do learn the layout. It is nearly impossible to use an iPad without learning the layout of every app so you can just reach for the spot where you know the control or panel you need is and usually touch very close to it. Touching the wrong spot is harmless, but it can quickly become frustrating.
There are rarely floating panels on an iPhone, whereas multiple pains are common in iPad apps. You can never trust that you will find everything in an iPad app by just swiping around. On the phone this usually works fine.
The E-mail interface is just one excellent example. On an iPhone you are either in the list of messages or in an E-mail message. If you're in a message the back button or gesture brings you out of the message and in to the message list. When in the message list the back button or gesture brings you out of the message list and to the list of accounts or mailboxes. On an iPad multiple things will be on the screen at once and the back button is pretty useless and the go to top gesture won't consistently bring you to the top of what you were just reading.
The experience of swiping one way and then not finding the same item when you swipe back is far more common on an iPad.
From my experience and observation, typing on an iPhone is easier. I don't personally know any blind people who type with 2 hands on an iPad, so the big keyboard just makes everything harder to reach. The phone-sized keyboard that is an option for sighted users on iPads does not work with VoiceOver, for no explainable reason. At least the common problem with closing the app when trying to type a space seems to have been greatly reduced in the latest iOS version. There is also less trouble with the keyboard accidentally being dismissed or focus jumping when editing text with the iPad keyboard. This used to be a serious usability issue that made me hate my iPad.
The cameras an iPhones are usually better than those on iPads. Why would you care if you're a blind person? Because your iPhone can be your eyes in many situations, and it works much better if it is easy to hold and has flash. It can identify money, read text, even handwriting in some cases, and identify products and people.
You can ask Siri for a business name and then have it call it.
You can double tap on phone numbers in E-mails, messages, notes, and any other documents and cause the phone to call that number.
There is a built-in level in iPhones. This can help you position things straight. It can also help you level the camera for taking a picture.
It can help you get places, or back home with GPS apps and ride hailing services. Even without a cellular connection the compass in iPhones can be extremely useful when you get turned around and confused in buildings and outside in snow or large open spaces.
You will usually have a cellular connection so you can verify that you really are on the street someone told you you are on by asking Siri "Where am I?" It's amazing how often this is helpful.
The NFC technology in iPhones can be used for easily labeling and identifying things. It also allows for making payments with ApplePay. This is far more accessible and secure than any other payment method.
Recent iPhones can help you find your other possessions if you purchase AirTags to put on or in them.
It is easier to mute and unmute notification sounds on iPhones because of the little switch on the side. Just remember it won't silence alarms.
On iPhones tapping the back of the phone 2 or 3 times can both be assigned to a shortcut, phone feature, or VoiceOver command.
It is important to consider the size of your hands when buying an iPhone. I have small hands so I have a Mini iPhone, because that is easier for me to hold and use. If you have larger hands you would probably have trouble with a small phone because it would be harder to perform the multi-finger gestures that are necessary for using Voiceover, so you should purchase a normal sized or pro sized Phone.
No matter the size of the device you will need to be able to perform scrolling gestures with 3 fingers. At times there is a scroll-bar or another method for changing pages, but using VoiceOver is likely to be way too frustrating for you. Not being able to perform the gestures right away is normal, so I am only talking about a known problem with finger coordination. I am hoping I just lack knowledge, but my own testing suggests that touch accommodations that can be used to make iPhones and iPads easier to use for people with motor impairments mostly do not work with Voiceover.
summery:
If you either have usable vision or excellent spacial memory and you are not frustrated by complexity an iPad could be an excellent tool, but an iPhone is a better option for anyone wanting only one device or a less steep learning curve.