I have both in 29er (jn fact one of each on the bike at the moment), I don’t really notice any difference in weight so if you think you’re going to be hard on them then no good reason not to get the Arch Ex. The rim weight difference isn’t much compared to the tyre and a bit of clag.
That said, for southern softy Surrey Hills riding, Crests have always been fine too. I’m 80kg in birthday suit, not super aggressive.
One observation: I put 2.35 Hans Dampfs on these, they went on fine but the tyre profile is very bulbous (‘light bulby’ shaped), I decided I wasn’t happy with that from a burp risk POV so took them off. I’d say if you think you want to run bigger tyres than maybe 2.3 (check the Stans site for that), get a wider rim.
I’ve now got 2.3 Spesh Butcher Controls, they look and ride fine. I uses 2.25 Schwalbes too.
Edit: the Stan’s site says Arch Ex is 80 grams heavier than Crest in 29. x 2 (both ends) x 3 (effect of rotational mass at the end of a 29er spoke compared to static frame weight) = the equivalent in terms of effort of adding 480 grams, about a pound, of frame weight, if my figures are valid. Less effect in a smaller wheel size as the rim is closer to the axle.