I've learned to prefer doing both. I start with spending enough time with pen and paper/whiteboard to get a coherent picture of what I want to build and how, and then I start coding it. I flip between "design" and "code" whenever I start to feel that current activity is getting hard. Getting increasingly confused as to what functions to write, how, and where to put them? I switch to design phase and sort it out at a higher level. Starting to feel I'm losing touch with the ground, or that I'm not able to figure out which design option is better? I go back to code and write what I can until the code itself reveals answers.

In essence, I see creating software as a dance between higher-level, structural concerns and low-level, immediate needs. Since one mode solves the problems found in the other, I try to be working in whichever mode I can make progress most effectively, and flip as soon as that changes.