I think this applies to the mid-tier and lower, not the high end bikes that all have the op en groups.

I would bet if you took two stock bikes and added lead weights to the lighter to make them equal (which is done for UCI weight requirements) the originally lighter bike would be faster and ride better.
The lighter stock bikes tend to have more refined components that remove weight where it is less needed.

Its your money, but I'd generally go lighter weight as it is often better stuff.