The important aspect of buying a quality ruck sac, is to buy the very best you can afford, and if there is one particular model that is out of reach budget wise, SAVE FOR IT.
I have noticed that materials used today, are not as good as those back in the 1970 and 80s.
A very old Berghaus Dart 45, i bought donkey's years ago, is still in amazing condition, no tears to the fabric, zips still in fine working order, and even though its very faded, it will last another thirty odd years.
Choosing a larger capacity sac, in the 50-60L range, needs to be done carefully.
Quality will cost good money, but visit the websites of the leading manufacturers, and see what sacs are at the top of their range.
Do not be put off by the inflated prices, because you do not buy a sac every day, and chosen carefully it should last many years.
An expensive purchase at the time, will not seem so bad, in twenty or so years, when its still in fine condition, and giving good service.
Its ever so easy to buy a budget sac, thinking its more than i want to spend anyway, but if your prepared to save a few quid every month, that quality expensive sac, will pay for itself over the years.
Its ever so difficult to recommend a particular sac.
You know the capacity your after, and anyway, you have to try on the various sacs on your want list, to make sure their a good fit.
A sac is not like a pair of boots, that in a few years, will need replacing, its a bit of equipment that can last the owner a considerable number of years, get it right the first time.
If you do see a particular sac, but are put off by the price, save for it, it will be money well spent longterm.