The Nvidia vs ATi comparison is a good one. For a brief window of time, one of the two will be slightly ahead in terms of performance or production quality; and both companies have their bad times. But for an LC PDS NIC, any "big brand" will be good enough. The Sonic that I mentioned was used as an Apple reference design; somebody at Apple commissioned an LC PDS NIC for third party companies to copy.

If you want to research which card works best on paper, the important design parameter is cache size versus TCP/IP MTU.