I'll try to answer your question, because when I ordered my first alpaca cinch I was wondering the same thing and could find very little info.
I have two lpacas and several mohairs and they are very similar to each other. Really, it's more about getting a high quality cinch than which fiber it is, because I find them REALLY similar. I am currently in love with the Classic Equine Alpacas, but that is probably more to do with the the fact that it is a quality cinch than that it is alpaca.
There are only two slight differences I've found. 1. The alpaca seems slightly "fluffier," or like the fibers are finer, whereas some mohairs are a little more silky. But then I have at least one mohair that is also more fluffy. So I don't know how else to describe it. It's like mohair is silkier and alpaca is fluffier.
and #2. Sometimes the alpacas come in different colors whereas mohair is usually that creamy tan color. I have one alpaca cinch that is brown, and the other is creamy tan like my mohairs.
But basically, they are nearly identical. I have been VERY happy with the alpaca cinches from Classic Equine (the only alpaca cinches I have tried). I would find it hard to choose between alpaca and mohair, but if I were buying another cinch I would probably go with the alpaca just because I might get a unusual color. But fiber-wise, you really can't go wrong with either one. Alpaca seems as good or better than mohair.