My understanding of inserts generally follows this..
- Harder insert for smoother tracks
- Softer insert for bumpier tracks
- Harder insert for higher speed tracks
- Softer inserts for lower speed tracks
- Thicker inserts for cooler temps
- Thinner inserts for warmer temps
Presuming I am correct (others may have a different understanding as myself) but for the colder months i would opt for thicker inserts.
The theory here being that the thicker insert holds more temp in the tyre. The thinner inserts have a larger air gap between the rubber and the insert and allows for the tyre to stay cool (or cool down).
You probably want to work out which tyre/insert combination gives you the best grip and tyre wear, with one being a trade-off of the other.
I am still on the search for the answers here myself, so I'd be interested to hear other people's ideas!