It's twofold:
Chevre, IMHO, is the 'better' leather as it is not artificially grained. Chevre is also very durable and doesn't scratch easy.
BUT
If you order the bag, you don't know what it will look like eventually, as chevre does have the 'spine', and - again IMHO - sometimes the spine can almost disfigure the bag. It's definitely an acquired taste.
With Epsom however, there is no such risk. You know exactly what the bag will look like. Epsom is artificially grained, but very, very sturdy. I think it's a lot harder to scratch than its predecessor Courchevel. However, once it is scratched, the scratches will be difficult to get out, but then again, if you don't throw your bag around but treat it naturally, it will take a very very long time to get scratched.