‘Stiffness’ at the crank is going to be a combined product of the bottom bracket and the design of the crank, not simply the cranks material. I agree with the poster above about the difference (or realworld lacktherof) between 6061 and 7075. An external bearing, large spindle crank will be stiffer than a square taper. A wider crank, say like a dura ace 9100, will be stiffer than a slimmer crank, like a dura ace 7400. And these things will create extra stiffness well beyond what any difference between Al alloys may give you. If a crank is stiff but your bb isn’t, or your crank isn’t but your bb is, it’s a wash. I’ve never ridden ominums, but in my opinion, worrying about a minuscule amount of weight is a pointless endeavour, as you could gain or lose more than a minuscule amount of weight if you cycle after a heavy meal. I dont know how stiff ominums are, but stiffness is more important than weight for a crankset, if performance is what you’re concerned with. your current cranks are square taper, so I have no doubt that ominums will give you much better performance and you won’t notice at all, that minuscule weight. I say this as a person who uses a DA 7400 crankset and 7400 square taper bb.