I'm pretty sure the extended free trial to level 60 actually hasn't been implemented yet. As far as I know, that's happening with patch 5.3's release on August 11.

As for the free 30 days, I believe that starts once you put in your game code and log into the launcher. Also note that once you purchase the full game and activate your code, you're no longer a free trial account and will not be able to use the free trial at all, either the current L35 version of the extended L60 version in a couple of weeks, even if your sub lapses.

If you want to get the most bang for your buck and you're okay with the various restrictions free trial accounts have, then absolutely wait a couple of weeks and take advantage of the L60 trial. That's an insane amount of content for a free trial.