I think it depends what you use them for. Personally, since I use my external monitor for coding, I'd rather optimize for PPI. I love reading code on my MacBook's Retina screen, which is about 218 PPI, and I definitely notice the difference when reading code on non-retina. Unfortunately it's extremely expensive to get the equivalent of Retina PPI in an external monitor. But you can still get decent readability even without Retina PPI if you avoid issues like display scaling. Here's a great blog post [0] about the tradeoffs of screen size, resolution, pixel density, and the "zone of despair" where display scaling causes problems.