Legendary Gratias Ring VS Angelic Blessing

I thought I read somewhere that the effectiveness of more attack goes up depending on how much %DEX you had? How much %Dex would you need to really notice a difference?

It doesn't matter. The effectiveness of more crit also goes up as you get more %DEX. The only way you'd ever be better off using the 2att instead of a crit ring would require you to have a ridiculously low total attack. I don't have time to run the numbers, but it really is a nonissue, you should get the crit ring.