There will be a million or so opinions here on your questions, so I'll go early.
Cowhide is not as hard as horsehide. The stitching on a horsehide will wear versus the hide because the stitch will not pull down into the hide quite as much as cowhide. All of mine are cowhide, but some think horse handles the sweat and heat better. Here's a better discussion from another site:
If you carry between 3 and 5, you'll want a 15 degree cant or more typically, although everyone has an individual preference. It is very dependent on your body shape, size, length of waist, arm length, etc. QK Shooter has a good schematic on cant selection. The further around the holster is, the more cant you want.A lot has been said about horsehide as a holster material, and there are several very competent holstermakers using it in their products. However, I’d like to point out something: have you ever examined a really worn out holster? More often than not, it is the stitching that gives out, way before the leather does. One of the beautiful qualities of premium cowhide is that it molds up very firmly, yet is still fairly flexible. It is also just soft enough to allow the stitching to be pulled tight below the surface, where it is protected from abrasion. Horsehide is so hard that the thread sits on the surface of the holster where it is easily damaged. For this reason, I believe that a holster properly constructed from a premium cowhide is actually going to last longer than a comparable one made of horsehide. The other consideration that favors the use of cowhide is the issue of comfort. A holster’s job is to act as the “interface” between a block of steel and the human body. A holster made from cowhide will “break - in” and soften up just enough to mold itself to the contours of your body, which greatly enhances the level of comfort. Horsehide is almost as hard as Kydex when it is new, and will remain so for a very long time. The only solution is to oil it, which makes it too soft to properly support the weapon and makes an oily mess on your clothing. In the defense of horsehide, I will say this: The available supply of truly premium cowhide is rapidly dwindling. If it becomes unavailable, I will certainly employ horsehide rather than use a poor quality cowhide.
Here's a great resource on holsters, and a site for fantastic leather: http://www.delfatti.com/webdoc20.html