I posted before about opinions comparing the Dell 24" and 27" displays. In an effort to figure out which display would be better, I purchased both the Dell 24" (P2415Q) and 27" (P2715Q) displays to compare them and see which one would be better as an external monitor for my MacBook Pro Retina 15" (planning on using the one I don't choose for hacks and projects). Questions Will either the 24" or 27", with lower pixel density (PPI) than the MacBook Pro Retina 15", have a noticeably lower PPI (i.e. appear less 'retina')? Does scaling to a larger resolution than "Best for display" result in a lot of blurring? Will the 27", which fits the same amount of pixels on a larger area, have a noticeably lower PPI (i.e. appear less "retina")? Will apps look way too big on the 27" because they'll get blown up? Is the additional 'space' (area-wise) on the 27" make it worth the trade-off of lower PPI? Any performance problems with running each display? Review materials MacBook Pro Retina 15" Late 2013 Intel Core i7 2.6 GHz NVIDIA GeForce GT 750M 2048 MB Mini DisplayPort to DisplayPort connection Reference: Video – Costa Rica in 4K; Text – Instapaper; Web: The Verge, MacRumors Display configurations Main: Best for display (2x Retina exact; 1920 x 1080) Scaled: 2560 x 1440 Scaled: 2304 x 1296 Review 1 — Dell 24" Display Holy crap, this thing is beautiful. Any worries about the lower PPI leading to a lower quality Retina experience – nope. This thing is buttery smooth. Text is incredibly crisp and videos, especially 4K videos, are gorgeous on this screen – likely a testament to the IPS panel in this display. However, UI elements are a bit large but they're not out of control. The display is a good size and offers a decent amount of extra space compared to the MacBook Pro's screen. On the other hand, scaled resolutions do produce a noticeable amount of blurriness – text isn't quite as crisp and things just seem a bit muddy. [EDIT: pastrychef suggests turning off LCD font smoothing when using scaled resolutions by going to System Preferences > General, which helps a good bit in making the text less fuzzy and less bold.] Question 1 – Noticeably lower PPI? – Nope. Looks just as good as my MBP. Question 2 – Scaling resolution performance – Noticeable blurring. A little annoying, but the annoyance dissipates with use. Questions 3–5 deal with the 27" so they're skipped. Question 6 – Performance problems? – Some lag when driving the scaled resolutions at 1440p or 1300p. Review 2 — Dell 27" Display This thing is huge. Like blocking-the-sunlight-from-my-window huge. It's only 3" more than the 24" but it's noticeably larger. Watching the Costa Rica 4K video was pretty awesome on the 24", but it was unreal on the 27" – much more immersive. However, UI elements are slightly even larger than the 24" as expected, though I've been using this one for hours and haven't noticed any negative effects from UI being that large. I generally prefer large UI, however, so this might just be me. I find that I can fit many things on the screen, however, which is pretty nice. Scaling does introduce blurriness, though again, the scaling isn't too bad. The main side effect is 1) text is a little less crisp and 2) scaling makes performance a bit slower, adding a bit of lag which I've found annoying. The standard resolution for 27" displays, 2560 x 1440 (1440p, which matches the Thunderbolt Display's 27" resolution and adds 77% more screen real estate), looks pretty good. 2304 x 1236 (1300p) is probably ideal for me if I were to use a scaled resolution. Overall, the 27" adds more screen real estate without sacrificing Retina at 2x resolution, and the scaled resolutions are pretty damn good. Question 1 – Noticeably lower PPI? – Nope – almost as crisp as the 24" (especially when pushing it an inch further from my eyes). Question 2 – Scaling resolution performance – Noticeable blurring. A little annoying, but the annoyance dissipates with use. Question 3 – Lower PPI noticeable with 27"? – See Question 1 Question 4 – Do apps look way bigger on the 27" vs. the 24"? – A little larger, but it didn't bother me too much. Question 5 – Additional space (display area) worth the trade-off? – I think so, yes. Both have the problem of blowing up the UI a bit, and the 27" a bit more, but not too much more that it's annoying as mentioned above. I lean toward this being more worth it. Question 6 – Performance problems? – Again, some lag when driving the scaled resolutions at 1440p or 1300p. Comparing the two I still haven't decided which one will be better for my use, but I'm leaning toward the 27". Reading and watching stuff is really enjoyable on this screen, and there don't seem to be many performance degradations when using this monitor. Retina (2x) looks sharp enough to cut glass on both displays. The scaled resolutions on both are a bit less crisp but they're still usable and you can always switch between resolutions when you need more clear text vs. space. So all in all, 27" is larger, costs only about $100 more, doesn't seem less crisp, and gives you a ton more space. I'll still have to take some time to decide, though. I'll continue to update this thread with more views as I continue to use them. Hope this has been useful, happy to answer any questions you have.