Any amplifier (even an opamp qualifies) basically swings an output voltage, using the power supply. The higher the voltage the more the swign available. When amps run off higher voltages, distortion decreases across the board.
Basically, with a PCI card, you can get +/-12V (so, 24V) from the connector itself. Try hard as you can, this is not possible on PCI-E.
This does not mean that all cards implement it! The SB-Live! (which is the last consumer card I ever bought) was PCI, but used 5V for the opamps. I'm not kidding. I had to hard mod mine and use at least +12V. The card did not allow me to tap the -12V line as they didn't have the finger for that voltage, or I would have used that. I don't know much about consumer cards, TBH, so won't comment.
As to best cards, I'll be talking about the absolute best, and we'll ignore the availability and price for a moment.