I did a 12 nighter on the Brilliance in mid October and the weather was still very nice (shorts and t-shirts mostly, a long sleeve shirt on deck at night). The seas were not rough at all. The crowds were not bad really anywhere as the summer season had passed. I would think you would find a similar situation with the September date you mention, with possibly even better weather. I can't compare it to a May sailing, but that date doesn't seem like it would be bad either. Pick the date that comes quicker. :-) Enjoy.