The weight comes from - Durability, volume, pockets, weather resistance, ease and quality of attachment fittings.
Lighter weight is going to compromise some or all of those, you're right to consider it a quandary. I went down this weight saving route and bought some Altura Vortex Ultralite panniers, they're awkward to fit, flap about when empty, don't close as neatly as the Ortliebs - it was a mistake. If I ever go four pannier touring again, I'll use them on the front and leave them there for the tour.
If you're four pannier touring, what's the all up weight, is saving a couple of kg really going to be noticeable? The biggest saving of course would be dumping the front panniers and rack, if that's a possibility. I'm toying with the idea of bikepacking style fork luggage, lower volume, but I could distribute the weight. I use Ortlieb Sport rollers as my main panniers, they're only 30ltr but I can add 10ltr stuff sacks to the top of each for very little additional weight (Though the sacks are not 100% waterproof) My classic Ortliebs are 25 years old, when they finally give up, I may replace them with the lighter variants, though having seen some, I wouldn't expect them to last as well. My experience of Carradice stuff is limited, other than the saddlebags. I had some Super C front panniers, though the shape meant I couldn't fit as much in as I could alternatives with the same claimed volume. I also had a bar bag, which I foolishly lost, I prefer the Ortlieb, but doubt there's a huge weight difference.
I don't know the answer, like for like I doubt there's a significant saving to be made, so it's a question of what you're prepared to compromise. Alpkit do some interesting looking options, I've only seen it in the shop so have no experience.
