I threw together this rough and ready comparison:
The value at the bottom directly compares the amount of screen space in mm square, for every gram the tablet weighs. As you can see the Sony is a distant 1st place, even compared to the iPad Air and Mini w/ retina. Give it's incredibly low weight of 270 gram, I've got to say I think this is an a superb feat of engineering. Especially given it's also waterproof, dual speakers, and has SD card support as well.
I do prefer the 3:2 / 4:3 aspect ratios so that's the only downside. Starting to think tablets are best in 3:2 and phones 16:10
You could argue that is has a lower DPI given it's "only" 1920x1200...but to put this into some perspective. That's not fair off the DPI of Apple's retina display on the iPhone 6. (283 for the sony compared with 326 for the iphones). I'd say ~ 300 dpi is the best compromise for all portable display sizes. Anything above is just e-peen.