I'd go for reliability, durability, utility, etc before lightweight... I can't say I've ever noticed much of a difference in weight between panniers, even holding in my hand, let alone strapped to a bike with ten pounds of crap in it. Waterproof versus flimsier, inexpensive brands will probably be the biggest difference, with inexpensive being lighter. I have just some inexpensive Voyager branded panniers I picked up on a whim from my LBS. They've held up for a few years of bike commuting, and seem to be lighter weight than other panniers I've handled. But they're also less waterproof and potentially weaker, though I've had no issues.

It just kind of reminds me of guys who get $55 carbon fiber bottle cages, even though the weight difference from that to the most basic alloy cage is less than a mouthful of water. lol