My typical snowy day

Hi Varsha,

Well  spotted! That’s an interesting point much debated on forums ...

You can say both but for me the difference is in the type of sitting you are doing . You will use ‘sur’ if you are quite upright like, 

Je m’assois sur un banc, sur une chaise, sur un tabouret (a bench, a chair, a stool).

In the context of the cosyness aspect required, ‘dans’ fits better, you would use it for an armchair too -

...dans un fauteuil 

Hope this helps