September would be more manageable, and noticeably less humid, but I wouldn’t rule out August on that account. I think the threat of rain for August is probably overstated too, but I guess your decision should be based on what you intend to be doing whilst here. Less humidity would definitely make trekking, sightseeing, and anything physical more tolerable, but if you were just planning on being in the city it might not be such an issue.