Coda takes a Camino—style approach to tab design, while CSSEdit’s is more Safari. I prefer CSSEdit’s personally, it’s clearer which tab is selected and it does it with less visual elements (or visual clutter). But there’s no reason why OS X should have so many different styles of tabs for documents. It’s one of the last few user interface widgets1 that has yet to be standardised, but one that is becoming increasingly popular.
Hopefully Leopard will feature a default tab widget for Cocoa that most people will adopt to use in their applications, and hopefully it’ll look something like CSSEdit’s elegant implementation.