Similarities: The both use the same DSP (EMU10K1). They both support EAX. Both have support for 4 speaker setups.
Differences: The Live! has a SPDIF CD input on the card itself, while the PCI512 doesn't. Current retail Live! cards have headers that will support optical I/O daughtercards, as well as the Live!Drive. Live!s are consistently updated through the Live!Ware program, unlike the PCI512. In addition, the following Live! models have outputs for speakers with digital inputs: Live! original, Value "Digital" OEM, X-Gamer, MP3+, and Platinum.
For the above alone, I'd spend the extra $15-$20 to get a Live! instead.