I have an iPad 3 (same resolution as the iPad Air), as well as a Nexus 10 (299 PPI), and I find the SP3's screen to be comparable in overall quality. I judge resolution based on how well text is rendered, and Windows 8.1 ClearType does an excellent job. I'd say that text is equal among all of the devices, unless you get out your magnifying glass.
Now, there's a big difference between the Desktop and Windows 8.1 modern apps. The latter is where the screen really shines, while the former is a bit of a challenge sometimes. The Desktop is a bit small with the hiDPI screen, something that Microsoft needs to fix (unless their plan is to get rid of the Desktop completely at some point), but I've found it entirely usable. There's a fix for Chrome that needs application, but once it's done then Chrome looks great.
Overall, I enjoy using the SP3 far more than either the iPad or the Nexus 10. It's just far more flexible and versatile, and the screen certainly doesn't hurt the experience, relatively speaking.