Without wading too far into the wealth of information available for the crassulae family, of which sempervivum, echeveria, sedum and kalanchoe are part of, I’ll mention a few differences between the look alike sempervivum and echeveria which hopefully will help you choose the right one for your gardening needs.
Both sempervivum and echeveria are nicknamed ‘Hens & Chicks’:
Sempervivum are the perennial sort with tiny tooth edged leaves. They show off their beautiful colours in colder temperatures and bloom to their death after which the chicks take over in the garden patch.
Echeveria are annual. They also produce many babies(chicks), but conversely the mother plant continues to live after flowering. Theirs are a smooth edged leaf which tend to be fleshier/chubbier than the sempervivum. Though the echeveria leaf colours do not become tinged with red in the cold, the shapes and styles of their rosettes are varied and beautiful.
For a planter or annual project both types of rosettes can be mixed or matched to create stunning visual displays.
Of course sempervivum are more suited to the perennial rock garden but don’t hesitate to overwinter the echeveria in the house in a bright sunny window. Neither plant requires much water and are easily propagated by division or by leaf cuttings.