So im gonna try to answer the original question of the first poster (from what i can remember after reading 8 pages lol).
like just about all plants, there is varying amounts of alkaloids depending on many factors including time of year, soil quality part of plant used ect. so the best way to figure dosage is to grind the sample so it is more or less mixed, and start VERY SMALL. i cant stress this enough.
In regards to whether there is a difference between datura and brugmansia, yes but very subtle, like hops in different brands of beer. however i have heard from close friends who have tried henbane and belladonna as well as daturas and brugmansias, they claim there are noticable differences.
In regards to what to try (not that you should try any) i would suggest henbane, as it has had the most use in early and mid age europe as a drug, where belladonna and daturas have only been used as surgery aids, witches brews and poisons.
the only experience i have had with tropane alkaloids is PILSENKRAUT, essentially what all germanic and easter european beers were before the discovery of asian hops.
if you are interested in making pilsenkraut here is a recipe.
http://www.erowid.org/chemicals/alcohol/alcohol_brew.shtml
you can also find experiences online. like i said potency of these plants vary so if you use 40g of plant material, dont drink it all at once and DEFINITELY do not expect the same experience from the same recipe if you like it and make more.
henbane is safer than the other plants because it generally has less variation in alkaloids plant to plant, and has a good ratio of different tropane alkaloids to result in a (more) pleasant trip that doesnt last as long or have as bad after effects.
i hope someone finds this useful, as i would not go head on with daturas brugmansias or belladonnas, but have had enjoyable usually lighter one night trips with pilsenkraut.
here is the wiki page on henbane
http://en.wikipedia.org/wiki/Henbane