<< Why SB never made a pci version of it is beyond me >>
Funny you mentioned that, because Creative did produce the AWE64 PCI as an OEM part for a while. Even with all of the advancements that the Live! cards have made, I still miss my AWE64 Gold ISA a LOT. I really liked the fact that it had its own onboard memory (mine was upgraded to 16MB total with an expansion module) for Soundfonts instead of using host RAM. The only reason I stopped using it was because of ISA slot limitations. Go fig....
Back to the main question, there are number of differences in sound quality between the AWE cards and the Live!.
The AWE64 series were essentially the same cards as the AWE32s with some minor modifications (the "64" actually comes from the number of voices it can support through the included software synthesizer; it still only handled 32 voices in hardware). The Live! upmixes all source signals to 48kHz with 8-point interpolation before outputing them. The Live! also supports true full-duplex operation. The AWE cards claimed to have full-duplex support but realistically (due to its drivers), it was only half-duplex. The Live! has a programmable DSP that is capable of accelerating up to 32 DS3D streams. In contrast, the AWE series provides no 3D sound acceleration. Finally, the AWE cards used 4W amplified preouts. The Live! cards delete this in favor of improved sound quality (no amplification present to distort the output signal).
With all of those advantages the Live! has though, the MIDI output on my AWE64 Gold still sounded better for whatever reason. Several other people will probably tell you the same thing.