Horsehide.
It's thinner then cowhide and it doesnt wear out as fast. The longer you use it, the softer it gets. Plus, horse leather won't wear blue finishes.
Horse hide, hands down, is the winner there. Breaks-in in seconds, last forever, and won't ruin finishes. Works great with the smooth side in better than cow hide, too.
Tom
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A "Miss" is the ultimate overpenetration!
I strongly recommend you check it out before making a decision...after reading it, I went with cowhide. This is from a leatherworker that has quite a few years of experience.