Wondering if I can get your opinion here?

I am trying to decide between getting my 11 year old son a WM Ultralite Super 20* bag which has a 59" shoulder width at 26 oz, and a Mont-Bell UL SS Down Hugger #2 - 25* bag which has a 53" inside shoulder width (stretchable to 70") at 28 oz.

WM is 2 oz lighter and rated for 5* colder, but am I correct thinking the MB will be warmer because it is more narrow, therefore less excess space to warm in the bag? It would seem since MB stretches to 70" shoulder width, we could layer another bag in it to get to lower temps, any thoughts?

Any personal prefernces between these bags?

Thanks

Chuck