^I think the real difference isn't toxicity, but the fact that with oxalic acid crystals the end user can control the acid concentration, for better or worse. A bottle of store-bought vinegar is acetic acid already diluted to consumer-safe levels (or at least I hope...I'd hate to think that all those years dyeing easter eggs in the stuff have left me with an acute case of "easter lung").

Still, it'd be interesting to hear results from people who have tried both types of acid bath.