How do I know whether or not dried mushrooms are as safe and as nutritious as fresh mushrooms?

Well, again I can answer from a Japanese perspective, having lived there for 16 years and eaten more than my share of shiitake and many of the other mushrooms that Japanese love to eat several times a week.

• NUTRITION: For a start, mushrooms don't lose any of their potency when they dry. So either dry or fresh is equally good for nutritional purposes (for the record, Japanese eat a lot more fresh than dry).
• TASTE: However, dried mushrooms are preferred by some people for their more concentrated taste.
• POTENCY BY WEIGHT: Of course, by weight you will get a lot more potency in 100g of dried mushroom than 100g of "wet" mushroom with all the water weight missing.
• SAFETY: I had never before considered the possibility of a health difference between dry and fresh, however a cursory glance on the internet dug up this article from the Health Canada Advisory website. Hmmm, it talks about the possible dangers of anaerobic activity in fresh mushrooms caused by the non-perforated sealing of fresh mushrooms in plastic wrap. You can read the article, but the money-line appears to be, "In the absence of oxygen, C. botulinum can grow and produce toxin before visible signs of spoilage appear." C. botulinum appears as if it can possibly lead to Botulism. Wow! If anyone else has insight on the safety issue, that would be great.