Both SAM and RPR average about 1 positional every 10-12 seconds. That's pretty standard for melee this expansion, although some jobs have more forced positional changes than others, as was discussed in a previous thread.

Most jobs are forgiving if you play them freestyle, which is the level that most comments in this thread seem to be weighing in at. NIN is the most ping sensitive choice, however, irrespective of skill.

Also, I wouldn't wait eight months to decide what melee to play. Just find the aesthetic you enjoy right now and start practicing. All melee jobs have roughly equal interquartile ratios, so the skill/performance distribution is very similar in practice. The most popular melee, SAM, has very similar player numbers in raid content to SMN, so there's no great barriers to entry here.