I think this is all a marketing hangover from the days when all bikes were steel and the tubing manufacturers needed a way to differentiate. IMO, there's absolutely no practical difference in terms of performance between various grades of steel. 853 might be "stronger" than 520 in absolute yield strength but both are so far above the demands of cycling that it doesn't matter, at all. Both will be plenty tough. 853 might save a quarter pound over 520, but who cares? Also, the idea that 853 might be "harsher" than 520 is strange to me. Surely things like chainstay lengths and tire selection are far more important than the specific tubing a bike is made out of when discussing compliance and comfort. I might pay more for stainless (maybe... I also think being concerned about rusting frames is a red herring) but that's it.