Personally, I prefer the non-spaced variant when dealing with one-lined css. Simply because grouping it visually in chunks makes it easier for me to skim through code.

Here I can recognize 4 chunks almost immediately:

  .error{ width:350px; height:200px; color:red; background:blue; }

While here everything gets mixed up due to its uniformed spacing:

  .error { width: 350px; height: 200px; color: red; background: blue; }