6dex 7det 42sks vs 45 crit

It comes down to how much you trust our statweights and how much you value skillspeed. In terms of pure weights the 270 alexandrian looks better, but personally I consider skillspeed that low to have a value of zero in practice. There's the issue with skillspeed being tiered and not actually shortening your casttime until you get a certain amount, and then there's also the practical issue that moving your GCD from 2.5 to 2.49 seconds, for example, has almost zero value in reality. For a 0.01 difference in GCD you'd have to be attacking a target for 10 minutes straight in order to move 1 GCD ahead of someone with a 2.5 second GCD, which is simply not practical.

Anyway I won't ramble on, it's frankly close enough that I don't think it matters either way which one you choose, someone could swap your rings while you were asleep and you wouldn't notice any difference.

But at the very least it's good information to know that in this slot the 260 shire ring is very close to the 270 alexandrian ring. It can help you decide what to spend pages on if, for example, you're trying to decide between buying the alex belt or alex ring with pages. Since we know that the 260 ring is a good substitute, you'd probably decide to buy the belt instead since that's a certain upgrade over any other belt item.