Grade 30 vs Grade 40 Chain ?

[klem]

You need to define the conditions that your chain will work under. Do you want to feel comfortable with the chain in 80 knots with 2 miles of fetch or would 40 knots and .5 miles of fetch be sufficient? The reason that I ask this is that G30 would be sufficient for the vast majority of anchoring situations but it may not be strong enough in severe conditions.

Unfortunately, it is very hard to predict loads but the ABYC has some guidelines that are probably on the conservative side but would be a good start.

There is nothing inherently wrong with G30 for anchoring, it just needs to be strong enough for all conditions you want your ground tackle to work in. Being in the middle of a huge storm is no time to find this out. As to seeing chain break, I have seen chain break and I have seen it elongate, strength does matter in the extreme.