Using all zeros will be faster, but using pseudo-random data will be more secure (but slower). How much slower...who knows (perhaps 1.5 to twice as slow). However, PRNG (pseudo-random number generation) is much more secure on a modern computer (i.e. a computer made from the year 2000 onwards). Depends on your needs: speed vs. security.
Thanks Duke! any reduction in the 3hrs. one pass PRNG time will help,as my security needs are modest-just a precaution really.I wonder how long a 35 pass Gutmann would take?!!! It all makes one wonder if these latest huge capacity HDDs are counter productive in this respect,as maybe those with a real need to wipe securely will just not bother because of the time factor.