The numbers 10 in the 1055 means it is a "Plain Carbon steel," while the numbers 51 in 5150 mean it is a "Low Chromium" steel. The last two digits show how many "points" of carbon the steel has. ( The more carbon, the better you can harden it, up to a point ) Steel Types, from ABANA Hammer Blows.
My guess is that the 5150 is a little more corrosion resistant than the 1055.
Hope this helps.
Ray