cotbed duvet 4.0 tog or 7.0 tog

[Dubbs]

Without stating the obvious, which one is preferable? I'm assuming as a winter duvet 7.0 tog is best but not sure?

I went for a lightweight duvet for dd.
Easier to add a sheet to it to make her warmer.
I didn't want her to get too hot IYSWIM

sounds like a great idea, was thinking that moving from a grobag to a heavy duvet might be too much, thankyou!