SDX900 is a 50Mbit signal. DVCPROHD is a 100Mbit signal. Compared to DV which is 25Mbit. As for the DigiBeta- that depend on what captured card is being used and at what data rate. In general until you get up to uncompressed HD, hard drive space is not really a problem with today's drives being inexpensive and vast.
Believe it or not, here's a Microsoft page with actual useful info(!):
http://www.microsoft.com/windows/windowsmedia/howto/articles/UnderstandingH...