Lemieux are definitely thicker and are proper lambskin, rather than wool sewn onto a cotton backing like the nuumed ones.
The thicker one might affect saddle fit though, so worth checking it'll be okay!
I much prefer the Lemieux personally.
Another vote for Lemieux, lovely good quality lambs wool. I found the Nuumed wool to be quite abrasive and cheap looking.
My friends horse had a terrible reaction to a Nuumed wool numnah. She used it once for an gentle hours hack. When she took the saddle off, her horses back was roasting hot, soaking with sweat and felt lumpy to touch. She hosed the area off with cold water. The horse itched itself red raw and she had to call the vet. Just an unfortunate allergic reaction to wool/lanolin. She contacted Nuumed who told her it couldn't be the wools fault, it must have been a dirty horse! Great customer care!
If you are going to use a wool product next to your horses skin, make sure its a good quality one and beware of possible reactions. Cotton is mostly best IMO.