I read a japanese study that showed evidence using chemical analysis that agarwood generally maintains its quality over time much better than sandalwood. In the specimens they tested, sandalwood chip quality degraded rapidly in comparison with agarwood chips, which held up amazingly well. Their conclusion was that sandalwood needs to be stored more carefully and used sooner. I cannot cite the source, but my memory is clear on this. I am storing some old sandalwood, but I figure I'd better enjoy it before it goes too far downhill. Frustratingly, I don't recall what time-scale was being investigated in the study.