There are two main types of greenhouse - lean-to and free-standing. A lean-to has the advantage of retained heat because of the insulating wall, but suffers from less light during part of the day when it is in shade. A free-standing greenhouse, if positioned correctly, will receive light all day long but looses heat more rapidly and needs more structural support. Both types of greenhouse come in two forms, glass from ground level upwards (excellent for tomatoes), or sections of wood or brick (better for propagation and flowers).
Aluminium greenhouses are certainly more durable than their wooden equivalents but the wooden ones are much better to look at and this is a significant factor in the garden, after all the whole point of this is to make your garden look great. Glass is the most common glazing material, but horticultural grade plastic is an alternative, perhaps more expensive but safer when children are around.