I would also go with October - we were there in December and it was also fine, just a bit of fog as the sun came up, so I think because it is so close to the equator, there is really little weather change. I hope you are staying overnight in the park because during the day there are lots of groups/people around but after the trains leave you will have the place more or less to yourself, also in the AM before the hordes arrive. The refurbished hotel there is nice - the train ride from Cuzco is interesting as well.