At Woorank we gave both of them a spin and a must say that for our team (8 people = 4 tech/2 CS/2 Allrounders working closely together) Trello works best, even with only 2 boards.

We've put up the Trello board on a big screen in the office and after some tweaking (eg. cutting the tasks in smaller bits) it's amazing how productivity rose, just because people are pushing the enveloppe for getting a ticket from the left to the right.

Also for a workflow of a ticket having several states (planned, in progress, done, reviewed, pushed, live), and going back and forth a lot, we just didn't find a nice way to do this in asana, without having lots of projects with several sub-lists, having to reconsider the workflow again and again, etc.

Asana looks more promising to me, when used in bigger companies (20+), having several departements, using there own projects or workspaces, with their own flows and way of working.