Beginning in the summer of 2014, I have taught using a digital whiteboard, a tablet supporting hand-writing in a note-taking app, projected on a screen with a projector (all of our classrooms have projectors, as well as a computer for instructor use, and inputs for instructor-owned devices). My solution until recently was a Microsoft Surface Pro 3 computer/tablet. This worked reasonably well. I used OneNote 2016 with a Microsoft Pen active/electronic stylus. The software and the stylus enable a good hand-writing experience: Only the tip of the pen writes, and if the fingers or the heel of the hand touch the surface, it does not interfere with writing. You can move the document around (to scroll it, say) using fingers.
But the Surface tablet was not always a smooth experience. Because it's a full Windows 10 device, it takes longer to come out of sleep mode, and a password must be entered. (Part of this is because any device owned by the university must conform to information security rules that require a passphrase.) Also, OneNote can be annoying. It's a very powerful piece of software, and it actually reads your handwriting as you write. You can use it to search for a given phrase you have written, and it can output your writing translated into typeset form, including mathematical notation. This is impressive, but the results are too rough to be of much use to me (I am happy to typeset documents using LaTeX, to ensure high-quality documents.) Worse, it can make the software a bit slow; it sometimes freezes for a moment or two, or seems a bit boggy.
Now I used the Surface tablet without much real problem for several years, but the device was getting towards four years old, and I figured it would be time for an upgrade. I happened to have some money from the university to spend on such things, and my thought was to get a new Surface Book computer: more powerful, with a larger screen. This would perhaps make it easier to use in class.
But a colleague of mine had a different solution: An Apple iPad, with the "Pencil" active stylus. This fellow had a relatively small iPad, but in trying the device with the Pencil, I quickly decided the experience with writing on it was at least as good as the Surface Book or other Surface products. So I decided to get an iPad, an iPad Professional, which is quite large (12.9" diagonal), and much less expensive than a Surface device with that size of screen. The software my colleague recommended for hand-writing and drawing was the GoodNotes app, which proves to work very well. I needed to make sure it exports what I've written in pdf format; it does. And the remaining thing I needed to determine was how to connect the iPad to the projector. An HDMI cable with an 'lightning'-to-HDMI adaptor was all that was needed.
So as of now, I have a very good solution for a digital whiteboard in the classroom: an Apple iPad Professional with an Apple Pencil, using the GoodNotes app. The device feels light; it is easy to work with (it goes to sleep and wakes up with little pause or trouble), and the experience with hand-writing is excellent. The visual quality and document output quality both compare to the Surface.
The only disappointment is that the image on screen is somewhat smaller than the Surface upon projection. This appears to be the result of a mismatch between the iPad (which outputs 1080p video) and the classroom projectors (a somewhat smaller resolution, I think). I'm tempted by buy my own 1080p video projector, as long it is an LED-lamp projector with a decent throw distance, to obtain a larger and higher-quality image in class. Not sure if what I want can be found for the price range I can handle.