It depends on how you expect that the weather will fluctuate.
If the temperature is fluctuating a lot, then you need the maximum versatility in layers. That probably means thin layers with a thicker layer over. That way, you can wear zero, one, or two thin layers.
For one high-altitude expedition, we knew that it was going to be pretty consistently cold, so I wore medium weight fleece tights with thick wool trousers over that. That went for a couple of weeks, and for most days I had goretex rain/wind pants on over that.