(We make checkoutapp.com)<p>We test, but we always put emotional arguments ("it looks better") above scientific data when it comes down to design. It's impossible to prove what gives better results in the long-term, but we would hate putting something online that doesn't look good to us. One thing I can really notice is that customers always tell us that our website/style stood out while they were evaluating the product, so it seems to work well for us.