They're both great...in different ways. The Viola is a VERY dark purple...darker than one might expect...almost black. The Cognac is a rich medium brown...more typical of croc embossed leather...you could also call it Pecan. So do you want something trendy or more traditional? They're both stylish.
I like Viola in the wallet. I have had all the colors but I gave the Turqouise and Emerald as gifts. The Viola I kept for me. It matches my Double Zippy and is a really sleek looking wallet. I think the Cognac looks a little more casual. And really for all intent and purposes it is a black color. Only under harsh bright light can you see purple showing through.