Which of the two types of hard drives (the old or the new) should be purchased to replace the company's damaged hard drive, knowing that the old one is more expensive and compatible with the damaged one, while the new one is cheaper and faster, and that the manager left the decision to my conscience?

If you took the hard drive without permission, or if you were given permission for it but it was damaged due to your negligence, then you are obliged to guarantee it. The guarantee is fulfilled by returning an equivalent item or paying its value, or paying what is agreed upon in a reconciliation. Therefore, you must buy a hard drive similar to the one you damaged, and it is permissible to buy a newer model if the owner or manager of the company agrees.