I had been looking into this issue as well prior to this post and actually contacted Montbell to see which was warmer. Montbell replied with the following:
"There is not going to be any real difference as far as warmth with those two jackets. What it really comes down to is pockets and fabric. The Ex Light does not have any pockets and the fabric is 7-denier nylon, so it is very
delicate. The UL Down does have pockets and the fabric is 15-denier nylon so it is slightly more durable. As far as the fill power, we use slightly less down in the Ex Light which is 900 fill power. That means that 1.8 ounces of 900 fill power (the amount in the Ex Light) is going to provide roughly the same amount of warmth as 2 ounces of 800 fill power (the amount in the UL Down Inner). The benefit of 900 fill power is it takes less of it to achieve the same amount of warmth, which in turn reduces weight."
Hope that helps everyone understand the two jackets.