I would rather go for PPI over total size. I had two spare monitors in my house to choose from when I got my new flying computer. The first was a 48-inch TV screen, but only 720 pixels tall. The second was a 32-inch computer monitor that was 1080 high. The difference was phenomenal. On the 720 screen you had to zoom in pretty far to clearly read the smaller numbers. But on the HD-1080p monitor you could zoom out a lot further and still have the detail.
So my advice is to go for whichever has the highest pixel count, preferably vertical pixels, those make a huge difference.