Hi,Jonna,
The main differences are habit, leaf shape and character of the flower spike.
The habit of
Stachys macrantha is to form a wide, bushy mound of foliage, with flowering stems extending slightly above the foliage. The flower racemes are short, and the individual flowers are quite large. The basal leaves are broad and heart-shaped. Here are some examples:
http://em.ca/garden/per_stachy...
http://www.levensgarden.com/20...
http://rhs.org.uk/Plants/50675...
By contrast,
Stachys officinalis has an upright habit, with long flower spikes of small flowers held on long upright stems. The leaves are strongly elongated and arranged at intervals up the long, upright stems.
Some examples:
http://www.findmeplants.co.uk/...
http://www.lejardindesophie.ne...
Even this compact 'Densiflorus' selection shows the same upright, long-stemmed character of
Stachys officinalis, though dwarfed:
http://www.songsparrow.com/cat...
Leaves:
http://en.wikipedia.org/wiki/S...
This article has more photos that show the differences and also compares them in a chart:
http://www.chicagobotanic.org/...
