Lizanne, both enameled and "bare" iron offer the same heat retention properties that make for excellent cookware. But it really all depends on the type of cooking you intend to do. While bare iron can be adapted to multiple cooking methods, enameled iron is really only suitable for long, slow cooking over low heat. The higher temperatures that bare iron is famous for withstanding will eventually cause the staining and darkening of the enamel that JerseyGal noted. While this can be brightened with good non-abrasive cleansers (like Barkeeper's Friend), I don't think it ever regains it's initial beauty.
All ironware is slow to heat. When you consider that adding layers of enamel to the iron, plus the fact that you cannot use extremely high heat on the enameled iron, you're looking at even longer heating time.
One thing you might want to consider about bare cast iron, as you say you are a sterile freak: detergents alone do not sterilize your dishes. Most detergents merely break down the surface cohesion of oils and water so that water can more easily penetrate and lift food particles and break down oils so they rinse away easily, but they do not sterilize your cookware. However, heating your cookware after washing, as is the preferred way of drying bare iron, DOES sterilize the cookware. So, even though most cooks do not use detergents to clean their bare iron, it is probably more "sterile" than your other cookware washed in detergent and towel dried or left in a rack. Just something to think about. As well, there are tons of fine cooks on these boards who will tell you that they wash their bare iron with detergent to no detrimental effect. Others, myself included, believe that detergents penetrate the seasoning and add a chalky, soapy taste to foods cooked in the bare iron later.
Another consideration is just how well you treat your cookware. Even the most careful cook is going to drop a dish now and then. When you consider the extreme expense of enameled iron and the fact that if you chip it, you've damaged it irreparably, in the long run it doesn't always seem like such a good investment.
Bare iron, seasoned properly, not only lasts the lifetime that you're looking for when you're considering heirloom cookware to pass on to your children, but what's more, unlike enameled iron, which can become dingy over time, bare iron actually grows more beautiful with age and use, as the seasoning builds into that rich, shiny black patina that is the mark of fine cast iron. Throw in the fact that the initial investment is just a fraction of the cost of enameled iron yet will give you a lifetime and more of excellent service, and I have to say, bare iron is the way to go.
If you are still convinced that enameled iron is the way you want to go, may I suggest you look toward the American-made Lodge brand, rather than the LeCreuset? LeCreuset is a d@mn fine product, to be sure, and the Lodge line does not offer as many varied pieces as LeCreuset, but what it does offer is comparable (I particularly like the apple pots--clever design!) and significantly less expensive.