You will be using your work desk a great deal so you should be comfortable throughout. Unfortunately, getting the right one can be tricky because comfortable positions are highly personal while the dimensions of these products are quite generic.
Mass market furniture is made with the average height of the population in mind. If you happen to fall outside this range, then you are likely to feel strained while typing on the computer. This is true whether you are sitting or standing.
Check the dimensions of the stand up desks, especially the desk height. Compare it to the height of other desks that you’ve been using to see if it’s a good fit. You could also get an item with adjustable height so you can sit or stand depending on your preference.
You can also get recommendations from ergonomics experts online. Take measurements as directed and make sure that you are relaxed with proper form throughout.