Its definitely a very personal decision. Depends mostly on what your desktop real estate needs are.
When traveling, air display (or similar for android) is amazing. I can always have an extra display when i need it for things like slack/email while my main display is kept uncluttered for work.
I have worked with USB powered DisplayLink Monitors and they are fantastic as well. Much larger than an ipad, and dont need to worry about charging it. I stopped using these b/c with destinations i can borrow spare monitor or hook up to a hotel TV via HDMI.
Have you considered just buying one or two inexpensive ($150) 24" samsung or similar monitors? If you buy one on sale, you can generally sell them on craiglist in 3-6 months for close to what you paid for it.
Here is an example: When I sign up for a booth at a conference, I order some inexpensive monitors and a 50+" LCD tv from amazon and have them shipped to the hotel. This ends up being considerably cheaper than renting AV equipment. Immediately after I arrive, I begin unboxing and photographing them to list on Craigslist with full disclosure of what we intend to use them for and when the person needs to come and pick them up with cash in hand for 10 to 15% less than what we paid for the item. By that evening all of them are accounted for without me worrying about running around town.
Unless you are producing 4k content, dont waste your money on 4k, or fancy monitors if this is a temporary solution. You are just wasting your $$.
Lastly, don't hesitate to ask on CL to borrow a monitor while you are in town. There are nice/honest people out there who may have one sitting around.