What you would be right in thinking is that any rebuilt turbo's have steel internals, I don't think any are ceramic. So if you have only one rebuilt you will have a mix! It's also possible given that both turbo's have seen a similar life that having had one go the other may not be far behind. So would you want to revisit the job or just get both re-furbished?
The consideration is how much it would cost to have both rebuilt (AET are pretty good) as averse to replacement with new. If you go new, do you want to go bigger? Which also means maybe new injectors, re-map, etc. etc. etc. If you go steel, standard you can at least turn the boost up a bit.
Pay your money, take your choice.
DaveG