There's no right or wrong answer here. It's a good idea to switch between the two at regular intervals, but if you want to focus on one or the other first it will work.
Personally, I advice to go with measurement/proportion first. I've found it easier to work this way because if your proportions are right, they are easier to dramatize. I know of people who had success the other way too though, so maybe it's worth asking yourself, what are you drawn more to?
I am with Jason on this. If you focus on the gesture, you will quickly gain an natural sense of the models proportion. Because as you focus on the action, your eye will automatically gauge the forms proportions.
If you focus on proportions, especially in the shorter quick sketches, you can easily lose the perspective and the figure will look distorted.
Personally i prefer to focus on drawing in blocks, define boundries that the whole body fits into (i rely heavily on negative space when i draw) and then divide it(proportions) then add skeleton and block out inner parts. It takes a long time so i barely get anything in 30 secs, so i just focus on proportions in this part, but in about two minutes i can get a decent figure.