Hi,
there is no definitive answer to this question.
It depends on the biggest variable of them all, namely the weather.
I would say that last year I took a standard type fleece, and at times I got quite cold in particular when it was being washed in the evening. The cold had a lot to do with the walking and tierdness though as opposed to the actual temperature.
I suppose it all depends on whether you are willing to carry the extra weight when it is not being worn.
I am going again on sept 4th and I will be taking a standard type fleece again. If I get cold I intend to wear my jacket over the fleece, jacket being a gortex shell lightweight.
Hoep this helps.
Peter