I'd go for stainless steel. I could imagine the chrome plated ones would fail faster with the chrome delaminating and flaking off which would murder the rest of the system. Steel balls tend to top out at grade 25.
If the hub/bottom bracket is high grade enough and you want the best performance you could replace the steel balls with ceramic as well. I wouldn't do this in an old worn system though, not worth while. You can get cheap ceramic balls at grade 25 which wouldn't be much different to steel, but last longer, but you can get right up to grade 3 which are very rare, and very very expensive.