Engine Cases - which ones are better?

[Stray Catalyst]

I've had H5 engines in the past, which are famous for the poor quality and low strength alloy used. My current daily driver has an H6 engine - a 1600SP from 1970. Is that alloy better than the H5, or is it the same thing? I'm not planning to rebuild this one any time soon - it still has less than 100k miles and is running strong, but I've got a few other cars available to me and want to know which engines are worth rebuilding, as opposed to 'when it breaks, throw it away'.