A, relatively heavier rucksack - when empty - might be superior to an empty ultralight rucksack. The key, I think, is which loaded setup is more COMFORTABLE.
I use an older Osprey Kestrel 38 liter rucksack - have done for some years now. It is among the heavier rucksacks when empty. But, even when filled, it fits like a glove, carries well, and I do not notice that I am wearing it after the first couple of days. In fact, when I am without it, beyond that point, I feel "naked."
I suppose my point is that the rucksack is an important piece of the overall kit configuration. Just because it is very light when empty, does not necessarily mean that it carries well, when loaded.
It depends - like most variable things in life.
In my experience, the two most most important items of gear are your walking footwear, and your rucksack. IMHO, all else flows from there and is of secondary importance.
If the footwear and rucksack are ill-fitted or not comfortable, it does not matter what the cost, quality, construction, cost, or weight of the rest of your gear is. You are starting at a performance disadvantage.
Hope this helps.
Tom