Is a standard British or standard American accent stronger?

You can't say that an accent is "stronger", it's relative. It's like if you were asking "Britain or America, which is farther?". It depends where you put your arbitrary reference point.

If at least you were considering all of British English versus all of American English, then we could say that in some sense British accent is stronger because it's always far from standard, the variability in Britain is much larger than in America.

I honestly am not even sure what a "Northern American," accent sounds like, lol.

I live in IL, and I've never met people who have an accent that have grown up here. In New York or Texas? Yea, I could see them having an accent.

American sounds stronger to me, unless Brits really try.

The American accent must be harder for Brits to understand than the other way around.

The divergence (and strength) is probably driven by "black English" nowadays.

American is always harsh on the ears

OMG...you can't be serious..what a git !...accents are SUBJECTIVE..!!!