If you want to use ETTL, you'll need it either on-camera, or off-camera with a ETTL sync cord. And ETTL can be very inconsistent. However, if you're moving around all the time, it's easier with ETTL than doing manual flash off-camera. On the other hand, if you can have a relatively static situation, then manual flash will give you much more consistent results, once you dial in the correct power level. I'd go for the ETTL flash to give yourself maximum flexibility. On the other hand, using either method, you'll have to practice until you start getting some consistent results.
For example, I took an impromptu portrait of my daughter before she ran out the door to go to work, and used a 580EX on the hotshoe, flash-head in bounce mode (off wall behind me) and exposure control under ETTL, to take some quick shots. Later in the day, my wife wanted me to photograph some artwork she was working on, and I set up two off-camera flashes using simple radio triggers and manual settings, to get even, shadowless light. The exposure for the latter was set using manual power levels, which I set after selecting the ISO and f/stop.