SXS is Sony Proprietary as is PS2(Panasonics) for DVCpro HD Panasonics AVCCAM (AVC intra )use SD cards
I know RED also have thier own memory card
None of these systems employ mpeg2 ( as used by tape) and each claim that the compression algorithms they use are far superior to HDV .. or their rivals format
Sony with its XDCAM format with EX using SXS : The XDCAM HD ( which uses proprietary discs) and even better HDCAM SR
Panasonic with AVCCAM using SD cards, The PS2 tend to DVCPro/ DVCPRo HD
The choice is very limted to these 2/3
AFAIK
The SXS cards are Express card interface however I dont see transfer by USB to a computer to be that limiting via SD card for AVCAM
At this level though there is a tendency to all sorts of transfer methods ( USB, Firewire Cardbus ect)
You would essentially be buying into a system and need to be happy with software support, not just the fact of a non tape based High quality format