Design
Both the Nokia X and Lumia 520 come with a premium polycarbonate design, and are easy to hold. Both these devices deliver what Nokia is known for, with good design and build quality, despite the affordable price point.
Winner: Nokia X
Display
Both these devices offer 4-inch WVGA (480×800 pixels) display churning out 223 pixels per inch (ppi). The Lumia 520 has got scratch resistant coating on top whereas it's absent from the Nokia X. Lumia 520 has an edge here.
Winner: Lumia 520