I remembered seeing a survey last year on Gardener's World, I think - they tested lots of trees. I think this is it:
"These days trees are generally grown as a specific crop and sold to garden centres and shops. You can buy direct from the growers if you visit Christmas tree plantations and some even allow you to choose and cut, or dig up, your own tree. If you buy direct, the tree will be at its freshest - and will therefore last longer in the home. This is the most environmentally friendly way of getting a tree as you can pot it up, or plant it out in the garden and reuse it.
If this isn't possible, choose one that is labelled as grown in Britain by a member of the British Christmas Tree Growers Association (BCTGA), as this should provide some assurance that the tree is of good quality and freshly harvested.
Varieties of Christmas tree
Norway spruce - the traditional choice and usually the cheapest. It has fallen from favour in recent years, as it tends to shed needles quite heavily. Nevertheless, if well watered and kept away from radiators it will drop fewer needles.
Nordmann fir - the most popular tree these days due to its needle-holding qualities. It has strong, straight branches clothed with thick, flat needles with a silvery underside. But it's the most expensive as it's much slower growing.
Fraser fir - this has a good shape and excellent needle-holding qualities.
Blue spruce - this tree has a silvery-blue colour to the thick needles and an aromatic, citrus scent.
Scots pine - a little more unusual, but a wonderful, bushy tree, with extra long needles that don't drop as readily."
Hope that helps!