To the questions: "Is external interface better than pci cards?"
The correct answer is "No."
Were I to use (just to stay in the same manufacturer)an RME HDSP9652, a Digiface w/PCI interface, or a Fireface 800 (three different ways to go) there is no 'better' until you begin to define what you mean by 'better'. You've listed two concerns... processing speed, and the sound of the unit, erroniously ascribed to the converters. You cannot mate that entirely with 'converters', as so many devices use the same converters yet sound radically different; and many devices have, over their manufacturing lifespan, used more than one model ADC or DAC at different times in different production runs.... yet you never hear people talking about that special "Terbo Crumpus from the Jan 2008 production run, which used the Sosumi 5435 ADC...". The associated analog circuitry plays a much larger part in the sound of a unit than is typically accepted/understood.
Yes, you can insert your personal concerns about what constitutes "Best", but without the introduction of other factors to the orignal question, the correct answer remains, "No.".