725 is also very good and seems to be the modern equivalent of 531.
I suspect that the problem with 953 is that the tube sections are optimised for road bikes and that therefore it'll be a bit fragile. There were a fair few 753 one-off MTB frames made in the early days, but the tubes were always a bit thin and the bikes tended to get wrecked after a really big stack.
There are a whole load of new non-rusting steel alloys around now which don't have the cracking problems of stainless. Corten is one of them - it's used for Sea Containers.
Ride it or milk it!