You all have your political reasons for picking but mine was more personal. As a dark elf, I originally wanted to side with the Empire, but then I met my npc fiancé, Jenassa. She was a dark elf too. She fought alongside me against many foes before I tried marrying her. When we tried getting married, a random dark elf dead body (glitch) showed up in the Temple of Mara, which freaked everyone out. (including Jenassa). From that point on she wanted nothing to do with me. So I swore to betray my own race and join the Stormcloaks.