I don't know of any reason not to eat wild dandelions. Garden cultivars are, as you say, the same species, just strains selected for size, thickness and tenderness of leaf and reduced bitterness.

'recently I heard a horticulturist say that you shouldn't eat wild ones because they have some unhealthy substance...'

Ah, but was the horticulturist right?

I'm a horticulturist too and we can be just as wrong as the next person.

Here in Australia we have the false dandelion - Catsear - which is slightly furry, flat and has stems with multiple flowers. It is blamed for the condition in horses known as Australian Stringhalt which is when a horse suddenly jerks up its hindleg very high, to its belly. You don't see it very often thank goodness, as it is incurable. (You do, however, see Catsear growing all over the shop, especially in times of drought.)

The true dandelion has smooth leaves, one flower on each stem, and the stem is hollow. It is edible in all its parts.

I can't help but think that your friend is confusing the two.

Thanks for the input. Now that I think about it, I think it had something to do with the latex. Does the more domesticated variety have less latex and thus less bitterness?

Anyway, I plan to eat both (wild and domesticated) in the spring and compare them.

Assuming you are correctly identifying your "wild" dandelions (and I have no doubt that you are!), as you noted in your post the culinary ones are the exact same genus & species.
However, wild carrot (aka Queen Anne's lace) is exactly the same genus & species as the garden carrot, ditto wild and garden parsnips. However, as floral_uk pointed out, the domesticated cultivars are much more palatable than the wild versions thanks to patient breeding by seedsmen.

Bu all means, try the French dandelions and compare them in growth habit and taste to your wild ones and vive la difference! :-)

OK, so here's a follow-up question: If I plant the domesticated French variety and after it flowers scatter its seeds throughout my garden, will it eventually be like a perennial and crowd out the wild dandelion?

Taraxacum officinale is indeed the dandelion species. Do take care. Some of the seed catalogs sell "dandelions" that are actually Cichorium intybus (chicory). They do look much similar when young but as chicory ages, it looks different. Leaves can become less dandelion-like and it is a biennial instead of a perennial like dandelions. When it bolts, it has pretty cornflower blue flowers. Personal experience. :)

As far as wild vs cultivated? My money is always on wild. They are oft times better suited to survival than pampered garden varieties.

While talking about eating dandelions, I'll pass along what an Iroquois herbalist said about eating wild dandelions while leading an herb walk I was on. She said to look for dandelions with white at the base of the leaves not red. She said the red ones were bitter and one of those in the pot would ruin a batch. (I personally find them all bitter. *grin*)

FataMorgana

In reply to your second question, farm_boy, I would bet that once your French dandelions escape cultivation they will be as "weedy" (coarser leaves, woodier roots) as the ones in your lawn. There is also the fact that your local pollinators will not distinguish among them, and so your cultivated ones, left to flower, will have a mixed ancestry. Again, I believe the results of such seeds would be a reversion to the lawn weed state. But this is just my (semi-)educated guess.