I certainly don't think chromexel exceeds CHROME TANNED horsehide in durability. Vegetable tanned horsehide is another matter and clearly inferior, but White's uses chrome tanned horsehide for the tops of its boots and vegetable tanned cowhide for its soles and shanks, as I understand it. Chrome tanned horsehide used to be very common. particularly in Europe, and it is still very popular today. It has fantastic characteristics. Shell cordovan is still one of the most highly prized leathers because of its great characteristics, including durability and beauty. I have had many item made from horsehide, and cordovan in particular, and they can go for years of hard use and still look like they are a month old. Horsehide is not good for some uses because it does not take a rigid form well when chrome tanned and is not at its best when vegetable tanned. That's why companies like White's use the chrome tanned iteration when building the tops of the boots and the vegetable tanned cowhide on the soles, heels and shanks. Chrome tanned horsehide also does not tool well, so it is not a favorite for uses where fancy tooling is nice. For such things as holsters, knife sheaths, many saddle and other tack pieces, and places where cowhide is better suited, horsehide may not be the best. but for the tops of boots, jackets, coats, pants, and a variety of other uses, no leather is superior to horsehide. In gloves there are many materials that can be used, depending on the purpose. Goat skin, deer skin, and calf skin and other more pliable leathers are usually the best to provide dexterity. For working gloves, the toughness of thin goat skin is hard to beat. For archery gloves, cordovan tips on the string fingers are wonderful because of the slickness than provides a good release and the durability that causes the rest of the glove to wear out when the cordovan still looks almost like new, even though it has been the most highly stressed and abraded. Although this link is about holsters, it does discuss the relative advantages and disadvantages of horsehide versus cowhide to some extent. http://www.holsters.org/which.htm
Having received my horsehide SDs, I can attest to the high quality of the leather used by White's. While I'm sure the oiled cowhide leather is designed to be the toughest of all the leathers for the boots White's produces, partially because of its thickness and tanning process, I can't imagine that the chromexel can be as durable as the horsehide. However, chromexel is a very nice leather in my experience and is a good alternative for those who prefer its characteristics. I would not hesitate to buy a pair of chromexel boots if they were readily available in my size, but I would probably choose a different leather for any custom order I made. BTW, Russell Moccasin Company has used chromexel in its boots for many years and reports good things about it. Because most of the Russell customers tend to be hunters, their boots generally see heavy use, and the chromexel seems to hold up well in those harsh conditions. My Russell's are built with other leathers that I think are more durable than CLX. Unless it is supplied to them, Russell does not use horsehide in their boots, to my knowledge.