Pixel density goes a long way. The 635 has a 854x400 ppi while the 520 has an 800x400 ppi. Because the 635 is actually bigger, it gives the 635 a blocky, pixelated look. The smaller screen of the 520(and the 620) make them look sharper by comparison.
This does not mean the screen is horrible. Contrast is good, and blacks are reasonably deep. But compared to other budget phones, and even compared to the 520 the screen is mediocre.
There's a slight blue hue to blacks, so Nokia labeling this ClearBlack is pushing it a little. But for a budged phone screen it performs well. It uses an advanced IPS LCD screen that looks more vital and recessed than other low end IPS LCD screens. This gives the 635 a more immediate look.
Viewing angles are good, there's no real shift in contrast.
But there's no automatic brightness as the 635 lacks an ambient sensor.
Brightness isn't the best on the 635 and not quite as good as the 520. Also there's a dim area at the top of the screen that seems to be where the downward-firing backlight LCD's live.
The 635 houses a Snapdragon Qualcomm 400 processor and 512 mb of RAM though, which makes loading apps sluggish compared to other budget phones at the same price. Because of the newest versions of Android and Chrome, Microsoft's performance lead doesn't really exist anymore.
And because the 635 comes with Windows 8.1, this even emphasizes the resolution issue more, since you can fit more smaller tiles onto the screen.
Because of this, and the 635 screen actually being lower resolution than most other budget phones of the same size, it would probably be more beneficial to spend a little bit of extra money to buy an upgraded version.
Overall...the 635 is an upgrade in some ways, but in others it might be better to stick with the 520. If you're willing to sacrifice screen sharpness for a larger screen and slightly upgraded software, go for it. It may take a little of time to get use to the lack of physical soft keys, as the 635 uses software soft keys. Also, the 635 has no dedicated camera button, but this is just a lack of convenience more than anything.
The design of the 635 is more colorful and cute though, and still has a durable and strong feel that comes with Nokia.
It's what your preferences are.