Both are fine.
In theory powdered could be easier to oxidize but also easier to extract due to larger total surface area, while shredded/in pieces will oxidize slower but also take a bit more work to get every bit of alkaloid out when extracting.
In practice, its pretty much the same, even powdered MHRB has been shown to be kept fine for many years with no noticeable degradation (specially if you keep it cold and dry but even if not it's still fine). On the other hand, if you have the root in pieces/shredded, you can always just shredd it as fine as possible first and then extract it, and leave it standing for longer in acid or base, and your yields will be equivalent as powdered bark.
Also, do check the FAQ since it talks about storage and shelf life, and a bunch of other questions you will likely have when researching or doing extractions.
Be well